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Week 12 · Chapter

Chapter 12 — Patterns of Inheritance

Introduction to Biology Free sample edition

The module's primary reading · ~40–50 minutes · Objective 6 (apply the principles of inheritance — segregation, independent assortment, the product rule, monohybrid, test, and dihybrid crosses, and the extensions of Mendel (incomplete dominance, codominance, multiple alleles, sex linkage, linkage, pedigrees) — to predict the outcomes of genetic crosses as probabilities), with Objective 5 (describe the eukaryotic cell cycle and the ordered phases of mitosis, compute a mitotic index, contrast mitosis with meiosis, and explain how crossing over, independent assortment, and fertilization generate genetic variation, including the 2ⁿ count) carried forward · Read it first this week; the lecture, the slides, the tutorial, the quiz, and the lab all build on it.

In plain language

Last week's rules work for many traits, but not for every one. Sometimes a child plant is a blend of its two parents, and sometimes a person shows two versions of a trait at the same time. This week you meet those patterns, and you meet the ABO blood types, which come from a gene with three versions in the human population. You also learn why some traits are far more common in men, because the gene sits on the X chromosome (one of the two chromosomes that differ between males and females). Finally, you read a pedigree (a family chart that tracks one trait across generations) the way a genetic counselor does.

Why you already care about this

Picture a garden bed of snapdragons. A gardener crosses a red-flowered plant with a white-flowered one, plants the seeds, and waits. Every flower in the next bed comes up pink. Last week's rule says one allele should hide the other, so where did pink come from?

Now picture a family. One parent has type A blood, the other has type B, and their child has type O. Neither parent has type O, and for a moment the family wonders whether a hospital mixed up a test. Nothing was mixed up. Both puzzles have the same kind of explanation, and you can work each one with the Punnett square you already know.

Here is the good news. Every cross mechanic from last week still works: one allele from each parent, four boxes, fill them all, then count. What changes this week is one rule, the rule for what a heterozygote shows. Red-green color vision deficiency turns up far more often in men than in women, and by the end of the week you will be able to explain why.

By the end of this chapter you will be able to:

  • tell incomplete dominance from codominance with one question;
  • cross two pink snapdragons and give the ratio and P(pink);
  • name the three ABO alleles, count the genotypes and blood types they make, and explain why type O is common;
  • work a blood-type cross and explain how two parents who show A and B can have a type O child;
  • explain why an X-linked recessive trait shows in a male with one copy, and why a male cannot be a carrier;
  • work the carrier-mother cross and give the share of sons and daughters affected;
  • read a pedigree's symbols and make the two decisions in order.

Carry this sentence with you: the boxes stay the same; only the heterozygote's look changes.

Where we are

  • Already covered: Week 10 showed how meiosis deals one chromosome of each pair into every gamete, and Week 11 turned that deal into Punnett squares, the product rule, the sum rule, and the 3:1 and 9:3:3:1 ratios.
  • This week: incomplete dominance, codominance, multiple alleles and the ABO blood types, the sex chromosomes and X-linked recessive inheritance, a short note on linked genes, and pedigrees.
  • How they connect: you keep every step of last week's crosses and change only what a heterozygote shows or where a gene sits.
  • Where we're going next: Week 13 asks what a gene is made of, with the structure of DNA and how it is copied.

Week 11 is the chapter this one leans on hardest. There you learned that each parent passes exactly one allele of each gene to each offspring, the law of segregation, and that you find the odds by filling all four boxes of a Punnett square and then counting. You also learned two probability tools. The product rule multiplies the chances of two independent events that must both happen. The sum rule adds the chances of outcomes that cannot happen together. You will use all three ideas in every section below. From Week 10 you need one more: crossing over in prophase I swaps matching segments between paired chromosomes.

1. Spot a blend: incomplete dominance

Think about mixing paint. Red paint stirred into white paint gives pink, a color partway between the two. Some genes behave a little like that. In snapdragons, a plant with one allele for red and one allele for white has pink flowers. Neither allele hides the other, so the heterozygote looks like something in between.

Biologists call this pattern incomplete dominance.

  • Incomplete dominance is an inheritance pattern in which the heterozygote shows a blend partway between the two homozygotes, as when red and white snapdragons give pink offspring.

Writing the alleles needs care. Last week a capital letter meant dominant and a lowercase letter meant recessive. Here neither allele is dominant, so you write them as two different capital letters of equal weight: R for the red allele and W for the white allele. A red plant is RR, a white plant is WW, and a pink plant is RW.

Worked example — the parent cross. A gardener crosses a red snapdragon with a white one.

  • The red parent is RR, so every gamete it makes carries R.
  • The white parent is WW, so every gamete it makes carries W.
  • Every box of the Punnett square holds one R and one W, so the boxes read RW, RW, RW, and RW.
  • RR × WW gives RW in all 4 boxes, so 4 of 4 offspring, every one, are pink.

In words: a red parent crossed with a white parent gives a whole bed of pink flowers, because every offspring is a heterozygote and every heterozygote is a blend.

Misconception check. Students often think pink means the red and white alleles have melted together into a new pink allele. It is tempting because paint does mix for good. The cure is to remember that alleles keep their identity. A pink plant still carries one R and one W, and it can pass either one on unchanged, as the next section shows.

2. Cross two pink plants

Now the gardener crosses two pink plants. If the alleles had melted into one pink version, every offspring would be pink. They are not. Red and white both come back, and the counts tell you exactly how often.

Worked example — RW × RW. Each pink parent is RW, so by the law of segregation each one makes gametes with R or with W, half and half.

Punnett square for two pink snapdragon parents, with each box's genotype and flower color
R from parent 2 W from parent 2
R from parent 1 RR (red) RW (pink)
W from parent 1 WR (pink) WW (white)
  • Fill the four boxes: RR, RW, WR, and WW.
  • WR is the same genotype as RW, so the boxes hold 1 RR, 2 RW, and 1 WW, and 1 + 2 + 1 = 4.
  • Read the colors: RR is red, RW is pink, and WW is white, so the offspring come out 1 red : 2 pink : 1 white.
  • Compare the ratios: the genotype ratio is 1:2:1 and the phenotype ratio is 1:2:1. They match, because each genotype has its own look.
  • Find the probabilities: P(pink) = 2⁄4 = 1⁄2, P(red) = 1⁄4, and P(white) = 1⁄4. As a check, 1⁄4 + 1⁄2 + 1⁄4 = 1.

In words: half of the offspring of two pink parents are expected to be pink, a quarter red, and a quarter white. Under complete dominance last week, the phenotype ratio was 3:1 and did not match the 1:2:1 genotype ratio. Under incomplete dominance, the heterozygote has its own look, so the two ratios line up.

Misconception check. Students often write 3 pink : 1 white for RW × RW. It is tempting because 3:1 was the ratio for a cross of two heterozygotes last week. The cure is to count colors box by box. Pink is not a dominant phenotype; it is the heterozygote's own look, and only 2 of the 4 boxes are heterozygotes. The ratio is 1 red : 2 pink : 1 white.

3. Spot both at once: codominance

Now picture a coat made of two kinds of thread, red and white, woven side by side. From across a room the coat might look rosy, but up close every thread is fully red or fully white. Nothing blended. That is the second pattern.

  • Codominance is an inheritance pattern in which both alleles of a heterozygote are fully expressed at the same time, side by side, as in the AB blood type, which carries both the A antigen and the B antigen.

The human case lives on your red blood cells. Each cell can carry marker molecules on its surface.

  • An antigen is a marker molecule on the surface of a cell; in the ABO blood group, the A antigen and the B antigen are the markers that decide a person's blood type.

A person with type AB blood has red blood cells that carry the A antigen and the B antigen at once, both in full. An animal case is a roan coat, in which red hairs and white hairs grow side by side; each hair is one color, and the coat shows both.

Worked example — the one-question tell. You sort three heterozygotes with one question: is the heterozygote a blend, or both at once?

  • A pink snapdragon from red and white parents is partway between them. A blend, so incomplete dominance.
  • A type AB red blood cell carries the full A antigen and the full B antigen. Both at once, so codominance.
  • A roan coat has whole red hairs and whole white hairs side by side. Both at once, so codominance.

In words: a blend points to incomplete dominance; two complete versions side by side point to codominance.

Misconception check. Students often use incomplete dominance and codominance as if they meant the same thing, because in both cases neither allele hides the other. The cure is the tell. Pink is a blend, so it is incomplete dominance. A and B side by side on one cell is both at once, so it is codominance.

4. Count the alleles in a population: multiple alleles and ABO

So far every gene has had two alleles. A single person still carries only two alleles of each gene, one from each parent. But across a whole population, a gene can come in more than two versions.

  • Multiple alleles are three or more versions of one gene held in a population, though each person still carries only two; the ABO blood-group gene, with its Iᴬ, Iᴮ, and i alleles, is the standard example.
  • The ABO blood group is the human blood-type system controlled by one gene with three common alleles, Iᴬ, Iᴮ, and i, which together give the four blood types A, B, AB, and O.

Each allele has a job. Iᴬ adds the A antigen to red blood cells. Iᴮ adds the B antigen. The i allele adds neither. Iᴬ and Iᴮ are codominant with each other, so a person who carries both shows both. The i allele is recessive to both, which is why it is written in lowercase.

Worked example — count the genotypes. You list every pair of the three alleles a person could carry.

  • Homozygous pairs, two of the same allele: IᴬIᴬ, IᴮIᴮ, and ii. That is 3.
  • Heterozygous pairs, two different alleles: IᴬIᴮ, Iᴬi, and Iᴮi. That is 3.
  • Total: 3 + 3 = 6 genotypes.
  • Sort them into blood types: type A is IᴬIᴬ or Iᴬi; type B is IᴮIᴮ or Iᴮi; type AB is IᴬIᴮ; type O is ii. That is 4 blood types.
The six ABO genotypes, the antigens on the red blood cells, and the blood type each one shows
Genotype Antigens on red blood cells Blood type
IᴬIᴬ A A
Iᴬi A A
IᴮIᴮ B B
Iᴮi B B
IᴬIᴮ A and B AB
ii neither O

In words: three alleles give six genotypes but only four blood types, because Iᴬ hides i and Iᴮ hides i.

Misconception check. Students often think type O must be dominant because it is common. It is tempting because last week warned about the opposite slip. The cure is to read the genotype: type O is ii, and i is recessive to both other alleles. Type O is common because the i allele is common in the population, not because it is dominant. Dominant describes what shows in one person; it says nothing about how common an allele is.

5. Solve the blood-type puzzle

Back to the family from the start of the chapter: a type A parent, a type B parent, and a type O child. A type O child is ii, so the child received an i from each parent. That tells you each parent must carry a hidden i. The type A parent is Iᴬi, and the type B parent is Iᴮi.

Worked example — Iᴬi × Iᴮi. Each parent makes two kinds of gametes, half and half.

Punnett square for a type A parent and a type B parent who each carry a hidden i allele, with each box's genotype and blood type
Iᴮ from the type B parent i from the type B parent
Iᴬ from the type A parent IᴬIᴮ (AB) Iᴬi (A)
i from the type A parent Iᴮi (B) ii (O)
  • Fill the four boxes: IᴬIᴮ (AB), Iᴬi (A), Iᴮi (B), and ii (O).
  • Each blood type fills one box out of four, so P(AB) = 1⁄4, P(A) = 1⁄4, P(B) = 1⁄4, and P(O) = 1⁄4.
  • Check type O with the product rule: P(i from the type A parent) × P(i from the type B parent) = 1⁄2 × 1⁄2 = 1⁄4, which matches the one ii box.
  • Use the sum rule for a type neither parent shows: P(AB) + P(O) = 1⁄4 + 1⁄4 = 1⁄2.

In words: this couple's children are expected to be AB, A, B, and O in equal shares, and half of them are expected to show a type neither parent shows. The type O child is not a mix-up. It is one of four equally likely outcomes. A ratio is still odds over many children, not a promise about one family.

Misconception check. Students often think a child's blood type must match one parent's. It is tempting because type looks like something you inherit whole. The cure is to remember that a parent passes one allele, not a blood type. A type A parent who is Iᴬi can pass the hidden i, and so can a type B parent who is Iᴮi.

6. Follow a gene on the X chromosome

Every human body cell carries 46 chromosomes in 23 pairs, as you learned in Week 10. In one pair, the two members can differ. These are the sex chromosomes.

  • The sex chromosomes are the X and Y chromosomes; a female carries two X chromosomes (XX), a male carries one X and one Y (XY), and the Y carries few genes.

A gene that sits on the X chromosome behaves differently from a gene on any other chromosome, because males and females carry different numbers of X chromosomes.

  • X-linked describes a gene carried on the X chromosome; its alleles are written on the X, as Xᴬ and Xᵃ, and a male's genotype carries only one of them.
  • Hemizygous means carrying only one copy of a gene instead of a pair, as a male does for every gene on his single X chromosome.

Here Xᴬ is an X carrying the dominant, unaffected allele, and Xᵃ is an X carrying the recessive allele. A female can be XᴬXᴬ, XᴬXᵃ, or XᵃXᵃ. A male is XᴬY or XᵃY, written with one X and never a second.

Think of a spare tire. A female has two X chromosomes, so if one carries a recessive allele, the other X usually carries a working dominant allele that covers for it. A male has one X and no spare. If his single X carries the recessive allele, nothing covers it, and the trait shows.

  • X-linked recessive inheritance is the inheritance pattern of a recessive allele on the X chromosome, which shows in a male with one copy but in a female only with two, so such traits are far more common in males.
  • Red-green color vision deficiency is a difference in color vision that makes red and green hard to tell apart; it is a standard example of an X-linked recessive trait and is far more common in men than in women.

How common? This course puts no number on it. What you learn is the direction of the pattern: the trait is far more common in men than in women, and the worked example below shows why.

Worked example — one allele, two sexes. You compare a male and a female who each carry one copy of the recessive allele Xᵃ.

  • The male is XᵃY. His only X carries the recessive allele, and there is no second X to cover it, so he shows the trait.
  • The female is XᴬXᵃ. Her second X carries the dominant allele, which covers the recessive one, so she does not show the trait.
  • A female shows the trait only as XᵃXᵃ, with the recessive allele on both of her X chromosomes.

In words: one copy is enough for a male and not enough for a female, which is why X-linked recessive traits turn up far more often in males.

Misconception check. Students often think an X-linked trait affects males more because the gene is on the Y. It is tempting because the Y is the chromosome only males carry. The cure is to name the chromosome. The gene is on the X, and the Y carries few genes. Males are affected more often because they have only one X and no second copy to cover a recessive allele.

7. Work the carrier cross

Take a mother who carries one recessive allele and does not show the trait, and a father who does not show the trait. What are the odds for their sons and daughters?

  • A carrier is a person who carries one recessive allele but does not show the trait because a dominant allele is also present; for an X-linked trait, only a female can be a carrier.

The mother is XᴬXᵃ. The father is XᴬY. Each daughter gets an X from each parent. Each son gets his X from his mother and his Y from his father.

Worked example — XᴬXᵃ × XᴬY. You list the mother's two gametes down the side and the father's two across the top, then fill all four boxes.

Punnett square for a carrier mother and an unaffected father, with each child's genotype and result
Xᴬ from the father Y from the father
Xᴬ from the mother XᴬXᴬ (daughter, unaffected) XᴬY (son, unaffected)
Xᵃ from the mother XᴬXᵃ (daughter, carrier) XᵃY (son, affected)
  • Fill the four boxes: XᴬXᴬ (daughter, unaffected), XᴬXᵃ (daughter, carrier), XᴬY (son, unaffected), and XᵃY (son, affected).
  • Sons: 1 of the 2 son boxes is affected, so 1⁄2 of the sons are affected.
  • Daughters: 0 of the 2 daughter boxes are affected, so 0 daughters are affected. One of the 2 daughter boxes is a carrier, so 1⁄2 of the daughters are carriers.
  • All children: 1 of the 4 boxes is affected, so 1⁄4 of all the children are affected, and that one box is a son.
  • Check with the product rule: P(son) × P(affected, for a son) = 1⁄2 × 1⁄2 = 1⁄4, matching the 1⁄4 of all children.

In words: in this family, each son has a 1 in 2 chance of showing the trait, no daughter shows it, and half the daughters are expected to carry it quietly, as their mother does.

Notice where the affected son's Xᵃ came from. His Y came from his father, so his X came from his mother. A son's X always comes from his mother.

Misconception check. Students often say an affected son got the trait from his father, or that a man can be a carrier. Both are tempting because last week every child got one allele of each gene from each parent. The cure is the X. A son's only X comes from his mother, so an X-linked allele in a son came from her. And a male cannot be a carrier: with one X, he either shows the trait or he does not.

8. Note linked genes

Last week's 9:3:3:1 rested on one condition: the two genes sat on different chromosome pairs, so they sorted into gametes independently. Some genes do not meet that condition.

  • Linked genes are genes that sit close together on the same chromosome and so tend to be passed on together; crossing over can separate them, more often the farther apart they sit.

Picture two beads close together on one string. When the string is handed on, both beads go with it. Two linked genes ride together the same way, so a cross that follows them does not give 9:3:3:1. The parental combinations turn up more often than independent assortment predicts.

Crossing over, from Week 10, is what can separate them. In prophase I, paired chromosomes swap matching segments. If the swap point falls between the two genes, the genes end up on different strands. The farther apart two genes sit, the more room there is for a swap between them, so the more often they are separated.

Worked example — two cases, compared. You compare two pairs of genes in an illustrative plant.

  • Genes on different chromosome pairs: they assort independently, so a cross of two double heterozygotes gives 9:3:3:1.
  • Genes close together on one chromosome: they tend to travel together, so the same cross does not give 9:3:3:1, and the parental combinations come out more often.

Honesty note: this section is a footnote on purpose. This course does not measure how far apart linked genes sit; that kind of mapping is beyond an introductory course. The idea to keep is the one above.

Misconception check. Students often assume linked genes still give 9:3:3:1. It is tempting because last week every two-gene cross did. The cure is to check the condition behind the ratio. 9:3:3:1 needs independent assortment, and linked genes sitting close on one chromosome do not assort independently.

9. Read a pedigree with two decisions

A family tree with names and dates is something many families keep. A pedigree is the biologist's version, built to track one trait.

  • A pedigree is a family chart that tracks one trait across generations, with squares for males, circles for females, shaded symbols for people who show the trait, and Roman numerals for the generations.

Each person gets a label: the Roman numeral for the generation, then a number for the person. II‑1 is the first person in generation II. Labels, not names, keep the chart about the trait.

You read a pedigree by making two decisions, in order.

  1. Dominant or recessive? If two unaffected parents have an affected child, the trait is recessive. Each parent carried the allele without showing it.
  2. Autosomal or X-linked? A trait far more common in males, passed from carrier mothers to sons, points to X-linked. Otherwise, the gene is on an autosomal chromosome.
  • Autosomal describes a gene carried on a chromosome other than the X or the Y, so that males and females both carry two copies of it.

Worked example — the worked family. The chart is described in words.

  • Generation I is an unaffected couple: I‑1, a square (male), and I‑2, a circle (female). Neither symbol is shaded.

  • Generation II is their three children: an affected son, II‑1, a shaded square, and two unaffected daughters, II‑2 and II‑3, unshaded circles. That is 1 + 2 = 3 children.

  • Decision one: two unaffected parents have an affected child, so the trait is recessive.

  • Decision two: the only affected person is a son of unaffected parents, a pattern consistent with X-linked recessive inheritance.

  • The mother: if the trait is X-linked recessive, the son's X came from his mother, so I‑2 must carry the allele. She is an obligate carrier, XᴬXᵃ, and the father, I‑1, is XᴬY.

  • A future son: this couple is the carrier cross from Section 7, so a future son is affected with probability 1⁄2.

  • An obligate carrier is a family member who must carry a recessive allele because of who inherited the trait from them, such as the unaffected mother of a son with an X-linked recessive trait.

Honesty note: a family this small cannot formally rule out autosomal recessive inheritance. Two carrier parents with an autosomal recessive allele could also have one affected son and two unaffected daughters. "Consistent with X-linked" is the honest wording, and a careful reader says so.

Misconception check. Students often read a pedigree by shading alone, counting the shaded symbols and stopping there. It is tempting because the shading is the most visible thing on the chart. The cure is to use the two decisions in order: first dominant or recessive, from the parents and their children, then autosomal or X-linked, from who is affected and who passed it on.

Pause and think, choosing one of these two prompts:

  • (a) Explain to a friend why the mother in the worked family must be a carrier, using only the fact that a son's X comes from his mother.
  • (b) Describe what one new child in this family would most change your read: a second affected son, or an affected daughter, and why.

Key terms

  • Incomplete dominance — An inheritance pattern in which the heterozygote shows a blend partway between the two homozygotes, as when red and white snapdragons give pink offspring. (first used: 1. Spot a blend: incomplete dominance)
  • Codominance — An inheritance pattern in which both alleles of a heterozygote are fully expressed at the same time, side by side, as in the AB blood type, which carries both the A antigen and the B antigen. (first used: 3. Spot both at once: codominance)
  • Antigen — A marker molecule on the surface of a cell; in the ABO blood group, the A antigen and the B antigen are the markers that decide a person's blood type. (first used: 3. Spot both at once: codominance)
  • Multiple alleles — Three or more versions of one gene held in a population, though each person still carries only two; the ABO blood-group gene, with its Iᴬ, Iᴮ, and i alleles, is the standard example. (first used: 4. Count the alleles in a population: multiple alleles and ABO)
  • ABO blood group — The human blood-type system controlled by one gene with three common alleles, Iᴬ, Iᴮ, and i, which together give the four blood types A, B, AB, and O. (first used: 4. Count the alleles in a population: multiple alleles and ABO)
  • Sex chromosomes — The X and Y chromosomes; a female carries two X chromosomes (XX), a male carries one X and one Y (XY), and the Y carries few genes. (first used: 6. Follow a gene on the X chromosome)
  • X-linked — Describes a gene carried on the X chromosome; its alleles are written on the X, as Xᴬ and Xᵃ, and a male's genotype carries only one of them. (first used: 6. Follow a gene on the X chromosome)
  • Hemizygous — Carrying only one copy of a gene instead of a pair, as a male does for every gene on his single X chromosome. (first used: 6. Follow a gene on the X chromosome)
  • X-linked recessive inheritance — The inheritance pattern of a recessive allele on the X chromosome, which shows in a male with one copy but in a female only with two, so such traits are far more common in males. (first used: 6. Follow a gene on the X chromosome)
  • Red-green color vision deficiency — A difference in color vision that makes red and green hard to tell apart; it is a standard example of an X-linked recessive trait and is far more common in men than in women. (first used: 6. Follow a gene on the X chromosome)
  • Carrier — A person who carries one recessive allele but does not show the trait because a dominant allele is also present; for an X-linked trait, only a female can be a carrier. (first used: 7. Work the carrier cross)
  • Linked genes — Genes that sit close together on the same chromosome and so tend to be passed on together; crossing over can separate them, more often the farther apart they sit. (first used: 8. Note linked genes)
  • Pedigree — A family chart that tracks one trait across generations, with squares for males, circles for females, shaded symbols for people who show the trait, and Roman numerals for the generations. (first used: 9. Read a pedigree with two decisions)
  • Autosomal — Describes a gene carried on a chromosome other than the X or the Y, so that males and females both carry two copies of it. (first used: 9. Read a pedigree with two decisions)
  • Obligate carrier — A family member who must carry a recessive allele because of who inherited the trait from them, such as the unaffected mother of a son with an X-linked recessive trait. (first used: 9. Read a pedigree with two decisions)

Summary

  • The cross mechanics stay the same; only what the heterozygote shows changes.
  • Incomplete dominance is a blend: RW × RW gives 1 red : 2 pink : 1 white.
  • Under incomplete dominance the phenotype ratio matches the genotype ratio.
  • Codominance is both at once: type AB shows the A and B antigens together.
  • ABO has three alleles, six genotypes, and four blood types.
  • Type O is ii; it is common because i is common, not because it is dominant.
  • Iᴬi × Iᴮi gives AB, A, B, and O, each 1⁄4.
  • A male has one X, so one recessive X-linked allele shows; a female needs two.
  • A son's X comes from his mother, and a male cannot be a carrier.
  • The carrier cross gives 1⁄2 of sons affected and 0 daughters affected.
  • Linked genes travel together and bend 9:3:3:1; crossing over can part them.
  • Read a pedigree with two decisions: dominant or recessive, then autosomal or X-linked.

Check your understanding

Work these before looking at the answers — they are ungraded, and the answers (with reasoning) are at the bottom of the page.

  1. In an illustrative flower, the alleles B (blue) and W (white) show incomplete dominance, and a BW plant has pale blue flowers. A grower crosses a pale blue plant with a white plant. Fill the Punnett square and give the color ratio and P(pale blue).
  2. A parent with type AB blood (IᴬIᴮ) and a parent with type O blood (ii) start a family. List the four boxes, give P(A), P(B), P(AB), and P(O), and explain why no child is expected to share either parent's blood type.
  3. For an illustrative X-linked recessive trait, a father who shows the trait (XᵃY) and a mother who is XᴬXᴬ have children. Fill the four boxes. What share of the sons show the trait, and what share of the daughters are carriers?
  4. In an illustrative pedigree, two unaffected parents in generation I have an affected daughter in generation II. Make decision one. Then explain why this trait cannot be X-linked recessive.

Answers (worked reasoning — read after attempting)

  1. The pale blue parent is BW and makes B and W gametes; the white parent is WW and makes only W. The four boxes are BW, BW, WW, and WW. That is 2 pale blue : 2 white, which reduces to 1 pale blue : 1 white. P(pale blue) = 2⁄4 = 1⁄2. The colors read straight from the genotypes because the heterozygote has its own look.
  2. The IᴬIᴮ parent makes Iᴬ and Iᴮ gametes; the ii parent makes only i. The four boxes are Iᴬi, Iᴬi, Iᴮi, and Iᴮi. So P(A) = 2⁄4 = 1⁄2, P(B) = 2⁄4 = 1⁄2, P(AB) = 0, and P(O) = 0. No child can get both Iᴬ and Iᴮ, because the ii parent has neither, and no child can be ii, because the AB parent has no i. Every child shows type A or type B, a type neither parent shows.
  3. The father makes Xᵃ and Y gametes; the mother makes only Xᴬ. The boxes are XᴬXᵃ, XᴬXᵃ, XᴬY, and XᴬY. Every daughter is XᴬXᵃ, so 2 of 2, all of the daughters, are carriers, and none shows the trait. Every son is XᴬY, so 0 sons show the trait. The sons get their X from their mother, who has only Xᴬ to give, so the father's allele cannot reach a son.
  4. Decision one: two unaffected parents have an affected child, so the trait is recessive. It cannot be X-linked recessive, because an affected daughter would need Xᵃ on both of her X chromosomes, and one of her X chromosomes comes from her father. An unaffected father is XᴬY and can pass only Xᴬ to a daughter, so she could not be XᵃXᵃ. The trait must be autosomal recessive.

Next up in this module: the Readings & Resources page, the lecture tutorial, the practice set, and the quiz; next week, the double helix shows what a gene is made of and how it is copied.

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