Lecture: Patterns of Inheritance
Lecture script · about 15–20 minutes spoken · Week 12
Opening
Picture a garden bed of snapdragons. Someone crosses a red-flowered plant with a white-flowered one, plants the seeds, and waits. Every flower that comes up is pink. Now picture a family. One parent has type A blood, the other has type B, and their child has type O. Neither parent shows the child's type. By the end of this lecture, you'll be able to explain both puzzles with the same tool you already own.
These ideas reach well past this course. A nursing student explaining a family's blood types uses them, and so does a future genetic counselor reading a family chart.
Here's where we are. In Week 10 you watched meiosis deal one chromosome from each pair into every egg and sperm. In Week 11 you turned that deal into Punnett squares, the product rule, the sum rule, and the 3 to 1 ratio. In this lecture I'll keep every one of those tools. Fill all four boxes, then count. That never changes. What changes is one rule: what a heterozygote shows, or where a gene sits. Next week, in Week 13, we open up the gene itself and look at DNA.
1. A blend: incomplete dominance
Think about mixing paint. Stir red paint into white paint and you get pink, a color partway between the two. Some genes behave a little like that.
Here's the term. 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.
Last week a capital letter meant dominant and a lowercase letter meant recessive. Here, neither allele is dominant, so we write them as two different capital letters of equal weight. R stands for the red allele, and W stands for the white allele. A red plant is R R, a white plant is W W, and a pink plant is R W.
Now cross a red plant with a white one. The red parent can only hand on R. The white parent can only hand on W. So every box of the Punnett square holds one R and one W. All 4 boxes are R W, and every offspring is pink.
One warning. Pink does not mean the alleles melted into a new pink allele. A pink plant still carries one R and one W, and it can pass either one on unchanged.
Pause and try this. A pink snapdragon makes gametes. What alleles can those gametes carry?
Think about what a pink plant is carrying inside.
Here's the answer. Half of its gametes carry R and half carry W, because a pink plant is R W and each allele keeps its identity.
2. Cross two pink plants
So what happens when you cross two pink plants? If the alleles had melted, every offspring would be pink. They aren't.
Each pink parent is R W, so each one hands on R or W, half and half. Fill all four boxes. You get R R, R W, W R, and W W. The W R box is the same genotype as R W, so the boxes hold 1 R R, 2 R W, and 1 W W.
Now read the colors. R R is red. R W is pink. W W is white. So the offspring come out 1 red to 2 pink to 1 white. The genotype ratio is 1 to 2 to 1, and the phenotype ratio is 1 to 2 to 1. They match.
Why do they match now? Last week the heterozygote looked like the dominant homozygote, so the phenotypes came out 3 to 1. Here the heterozygote has its own look, so the two ratios line up.
The probability of pink is 2 boxes out of 4, which is one half. Red and white are one quarter each.
Pause and try this. Someone says two pink snapdragons give 3 pink to 1 white. What did they get wrong?
Count the colors box by box before you decide.
Here's the answer. Pink is not dominant, so only the 2 heterozygote boxes are pink, and the cross gives 1 red to 2 pink to 1 white.
3. Both at once: codominance
Now picture a coat woven from red and white thread, side by side. Up close, every thread is fully red or fully white. Nothing blended.
That's 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.
What's an antigen? 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 carrying the A marker and the B marker at once, both in full.
There's an animal case too. A roan coat has red hairs and white hairs growing side by side. Each hair is one color, and the coat shows both.
So here's the one-question tell. Is the heterozygote a blend, or is it both at once? A blend means incomplete dominance. Both at once means codominance.
Pause and try this. A pink snapdragon and a type AB red blood cell are both heterozygotes. Which one shows incomplete dominance, and which shows codominance?
Ask the one question about each.
Here's the answer. The pink snapdragon is a blend, so it's incomplete dominance; the AB cell carries both markers in full, so it's codominance.
4. Three versions of one gene
Every gene so far has had two alleles, and a person still carries only two of any gene. But across a whole population, a gene can come in more than two versions.
That's 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 is the standard example. Its three common alleles are the A allele, the B allele, and a third allele written as a lowercase i.
Each one has a job. The A allele adds the A antigen. The B allele adds the B antigen. The lowercase i adds neither. The A and B alleles are codominant with each other, so someone carrying both shows both. The lowercase i is recessive to both of them.
Let's count. Pairs of the same allele give three genotypes: A with A, B with B, and i with i. Pairs of two different alleles give three more: A with B, A with i, and B with i. That's 6 genotypes in all. But they show as only 4 blood types. Type A is A with A, or A with i. Type B is B with B, or B with i. Type AB is A with B. And type O is i with i.
Here's a trap. Type O is common, so people guess it's dominant. It isn't. Type O is i with i, and it's common because the i allele is common in the population, not because it's dominant.
Pause and try this. A person has type O blood. Which two alleles do they carry?
Remember which allele adds no antigen at all.
Here's the answer. They carry two copies of the lowercase i, because any A or B allele would add its antigen and change the type.
5. The blood-type puzzle
Back to the family from the opening: a type A parent, a type B parent, and a type O child. Start with the child. Type O means two copies of i, one from each parent. So each parent must carry a hidden i. The type A parent carries A and i. The type B parent carries B and i.
Now fill the four boxes. The type A parent hands on A or i. The type B parent hands on B or i. The boxes are A with B, which is type AB; A with i, which is type A; B with i, which is type B; and i with i, which is type O. One box each. So each blood type has a probability of one quarter.
Check type O with the product rule. Each parent hands on an i with a chance of one half. Multiply them, and you get one quarter, matching the one i-with-i box.
Now add the chance of AB to the chance of O, by the sum rule. One quarter plus one quarter is one half. So half of this couple's children are expected to show a type neither parent shows. The type O child isn't a mix-up. It's one of four equally likely outcomes.
Pause and try this. In this family, what is the probability that a child has type AB blood?
Count the AB boxes out of four.
Here's the answer. One quarter, because only one of the four boxes holds both the A allele and the B allele.
6. A gene on the X chromosome
You learned in Week 10 that a human body cell carries 46 chromosomes in 23 pairs. One pair is special. Its two members can differ.
These are the sex chromosomes: the X and Y chromosomes. A female carries two X chromosomes, and a male carries one X and one Y. The Y carries few genes.
A gene that sits on the X is X-linked. We write its alleles on the X itself: an X carrying a capital A for the dominant, unaffected allele, and an X carrying a lowercase a for the recessive one. A male's genotype carries only one of them, because he has only one X.
Think of a spare tire. A female has two X chromosomes, so a working dominant allele on one usually covers a recessive allele on the other. A male has one X and no spare. If his single X carries the recessive allele, nothing covers it, and the trait shows. That's what 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.
Put that together and you get X-linked recessive inheritance: one copy shows in a male, but a female needs two. So these traits are far more common in males. Red-green color vision deficiency is the standard example. What to keep is the direction, not a number: it turns up far more often in men than in women.
Pause and try this. A male and a female each carry one copy of a recessive X-linked allele. Which one shows the trait?
Ask who has a spare X.
Here's the answer. The male shows it, because his only X carries the allele and he has no second X to cover it.
7. The carrier cross
Take a mother who carries one recessive allele and doesn't show the trait. A person like that is a carrier: someone who carries one recessive allele but doesn't show the trait because a dominant allele is also present. For an X-linked trait, only a female can be a carrier. Her partner is a father who doesn't show the trait.
Remember how each child gets the sex chromosomes. A daughter gets an X from each parent. A son gets his Y from his father, so his X has to come from his mother.
Fill the four boxes. The mother hands on either her unaffected X or her X with the recessive allele. The father hands on his unaffected X or his Y. The four children are an unaffected daughter, a carrier daughter, an unaffected son, and an affected son.
Now count. Of the 2 son boxes, 1 is affected, so half of the sons are affected. Of the 2 daughter boxes, none is affected, and 1 is a carrier, so half of the daughters are carriers. Across all 4 boxes, 1 is affected, so one quarter of all the children are affected, and every one of them is a son.
A son's X comes from his mother. And a man can't be a carrier of an X-linked trait. With one X, he either shows it or he doesn't.
Pause and try this. An affected son is born to this couple. Which parent gave him the recessive allele?
Think about which parent gave him his X.
Here's the answer. His mother did, because a son's X always comes from his mother and his father gave him the Y.
8. A footnote on linked genes
Last week's 9 to 3 to 3 to 1 rested on one condition. The two genes sat on different chromosome pairs, so they sorted into gametes independently. Some genes don't meet that condition.
Picture two beads close together on one string. When the string is handed on, both beads go with it. Linked genes are genes that sit close together on the same chromosome and so tend to be passed on together. A cross that follows them doesn't give 9 to 3 to 3 to 1.
Crossing over can part them. You saw it in Week 10: in prophase I, paired chromosomes swap matching segments. If a swap falls between two linked genes, they end up on different strands. The farther apart they sit, the more room there is for a swap, so the more often they're separated.
I'll be honest: this is a footnote. The idea to keep is that close genes travel together.
Pause and try this. Two genes sit right next to each other on one chromosome. Should a cross of two double heterozygotes give 9 to 3 to 3 to 1?
Ask whether the genes sort independently.
Here's the answer. No, because genes that close travel together, and 9 to 3 to 3 to 1 needs genes that sort independently.
9. Reading a pedigree
Many families keep a family tree with names and dates. A pedigree is the biologist's version. It's 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 generation's numeral, then a number. So the first person in generation two is called two-one.
You read a pedigree with two decisions, in order. First, dominant or recessive? If two unaffected parents have an affected child, the trait is recessive. Second, autosomal or X-linked? A trait far more common in males, passed from carrier mothers to sons, points to X-linked. Autosomal describes a gene carried on a chromosome other than the X or the Y.
Here's the worked family, in words. Generation one is an unaffected couple, a square and a circle, neither shaded. Generation two is their three children: one affected son, a shaded square, and two unaffected daughters, two unshaded circles.
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, which is consistent with X-linked recessive inheritance. If it is X-linked, the son's X came from his mother, so she must carry the allele. She's an obligate carrier: a family member who must carry a recessive allele because of who inherited the trait from them. And this couple is the carrier cross we worked earlier, so a future son has a one half chance of being affected.
One honest caveat. A family this small can't formally rule out autosomal recessive inheritance. "Consistent with X-linked" is the careful wording.
Pause and try this. Why must the mother in this family carry the allele, if the trait is X-linked recessive?
Trace the affected son's X back one generation.
Here's the answer. The son's only X came from his mother, and his father is unaffected, so the recessive allele had to come from her.
Closing
Let me pull it together in the order we built it. Incomplete dominance is a blend: red and white snapdragons give pink. Two pink plants give 1 red to 2 pink to 1 white, so the ratios match. Codominance is both at once, like the A and B markers on a type AB cell. The ABO gene has three alleles, 6 genotypes, and 4 blood types, and type O is common because i is common, not because it's dominant. A type A parent and a type B parent who each carry i can have a type O child, with a chance of one quarter. A male has one X, so one recessive X-linked allele shows in him. The carrier cross gives half the sons affected and no daughters affected. Linked genes travel together. And a pedigree is read with two decisions, in order.
Next in this module come the slide deck, the Readings & Resources page, the lecture tutorial, the practice set, and the quiz, then the discussion, the assignment, and the Blood-Type & Pedigree Detective lab. Next week, in Week 13, we open the gene itself and see how DNA is built and copied.
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