Exploring Incomplete Dominance in Heredity Beyond Mendel's Laws

1. Heredity • Chapter 10, part 2

2. Beyond Mendel’s Laws • Not all traits are controlled by single genes with dominant and recessive alleles. Other patterns of heredity involve: • Incomplete dominance • Sex-linked traits • Multiple alleles • Multiple genes (most traits involve this) • Chromosomal abnormalities

3. Incomplete Dominance • In incomplete dominance and co-dominance, both alleles are expressed in the phenotype. • incomplete dominance: two traits appear to blend in the heterozygotes. • co-dominance: both traits appear in the heterozygotes.

4. These snapdragons have two alleles controlling flower color: R1 (red) and R2 (white). Heterozygotes (R1 R2) have pink flowers. Unlike the case in complete dominance, the phenotypes show us which plants are heterozygous. R1R1 R2R2 R1 R2 R1 R2 R1R1 R1 R2 R2R2 R1 R2

5. mother In humans, a gene affecting hair texture (curly, wavy, straight) shows incomplete dominance. C1 C2 C2 C1 eggs father C1 C1 C1 C1 C2 sperm C1 C2 C2 C1 C2 C2 C2

6. The golden palomino horse is a cross between a white and a brown horse. This is another example of incomplete dominance: the colors appear to blend in the horse’s hairs.

7. The red roan horse has both white and red-brown hairs, while the blue roan has both white and gray hairs. The coat colors of both parents are expressed in the hairs. This is co-dominance. At the gene level, incomplete dominance and co-dominance are the same: in both cases, both alleles are expressed in the heterozygote. The only difference is at the phenotype level.

8. Solving single-gene (monohybrid) crosses with incomplete dominance. One hair color in cattle is controlled by a gene that produces red (R1) or white (R2) hairs. Heterozygotes (R1 R2) are roan. a. What color would the offspring of a red bull and a white cow be? b.What are the phenotypic ratios of a cross between a white cow and a roan bull? Both parents are homozygous. What gametes can they produce? A red bull (R1R1) produces these gametes: A white cow (R2R2) produces these gametes: R1 R2 R1R2 All offspring are roan and heterozygous

9. Solving single-gene (monohybrid) crosses with incomplete dominance. One hair color in cattle is controlled by a gene that produces red (R1) or white (R2) hairs. Heterozygotes (R1 R2) are roan. a. What color would the offspring of a red bull and a white cow be? b. What are the phenotypic ratios of a cross between a white cow and a roan bull? As always begin with the gametes each parent can produce. A roan bull (R1R2) produces these gametes: A white cow (R2R2) produces these gametes: R1 R2 R2 R1R2 R2R2 1/2 of offspring are roan 1/2 are white

10. W O R K T O G E T H E R • Pure-breeding red radishes (R1R1) crossed with pure-breeding white radishes (R2R2) produce purple radishes. • What is the genotype of the purple radishes? • What are the phenotypic ratios of a cross between a purple radish and a white radish?

11. Melvin takes pollen from a plant with red flowers and pollinates the flowers on a plant with white flowers. If color in these flowers shows incomplete dominance, what will the offspring look like? • All red • All white • All pink • Red or white, but not pink • Red, white, or pink

12. Sex Chromosomes Gender, however, is a complex issue, so saying XX=female, XY= male is an oversimplification. • In humans, genetic sex is usually determined by the sex chromosomes. • Typically, women have two X chromosomes, while men have an X and a Y (exception: the rare XY female, due to non-expression of the sry gene on the y chromosome during fetal development)

13. A Karyotype is an image of the replicated chromosomes in prophase. All chromosomes have matching homologues except the 23rd pair in males. Homologous chromosomes Paired sister chromatids The X and Y chromosome each have their own unique genes. To make a karyotype, a photo of a cell in prophase is cut apart and the homologues are carefully matched.

14. Sex-linked Traits • Traits that are carried on the sex chromosomes will show different genotypic and phenotypic rations in men and women. • The X chromosome has many genes, while the Y has only a few, so there are many more X-linked traits than Y-linked traits.

15. Women pass their X chromosomes to their children. Men can contribute either an X or a Y. Which parent determines the sex of the child? Mother or father? Can men be carriers of a recessive X-linked trait? No! If a boy has an X-linked trait, which parent did he inherit the trait from? Mother

16. When determining the outcome of a cross that involves an X-linked trait, we have to take into account how the two sex chromosomes are distributed in the offspring. female parent XA Xa eggs XA Xa XA XA Xa XA male parent Nettie Stevens was one of the first researchers to discover the patterns of X-linked inheritance. XA female offspring sperm XA Y XA Y Xa Y Y male offspring

17. This diagram illustrates a cross for an X-linked trait in fruit flies. Red eyes are dominant, white are recessive. female parent R r Red eyed carrier XR Xr R r eggs XR Xr R R r R R all the F2 females have red eyes male parent XR XR XR Xr XR female offspring sperm half the F2 males have red eyes, half have white eyes R r Y XR Y Red eyed XR Y Xr Y male offspring

18. Where does a boy inherit his X chromosome from? • His mother • His father • Equal chances that it could come from either parent.

19. Red-green color blindness is an X-linked trait. Charts such as these are used to diagnose red-green color blindness. They look very different for those with normal vision and those with RG-color blindness.

20. W O R K T O G E T H E R I have one brother who is red-green color-blind. Where did he inherit the color-blind gene from? Mom or Dad? What are the odds that I am a carrier?

21. Solving X-linked crosses. In sex-linked problems, we must track the gene and the chromosome that carries it. Red-green color-blindness is X-linked. If a man and woman with normal vision have a color-blind son: a. What are the genotypes of the parents? b.What are the odds of having another color-blind son? c. What are the odds of having a color-blind daughter? An X with a normal allele is shown like this An X with an affected allele is shown like this Xb XB The father is not color-blind so must not have an affected allele. His genotype is: The mother is not color-blind, but can be a carrier. Her genotype must be: XB Y XB Xb

22. Boys: 50% odds of being color-blind Girls: 0% odds of being color blind but 50% are carriers. Solving X-linked crosses. To set up the Punnet square, keep the chromosome and its allele together as one unit. Red-green color-blindness is X-linked. If a man and woman with normal vision have a color-blind son: a. What are the genotypes of the parents? b. What are the odds of having another color-blind son? c. What are the odds of having a color-blind daughter? Father can make these gametes: XB Y Mother can make these gametes: XB XB XB XB Y Xb XB Xb Xb Y Notice there are both girls and boys here, so we have to express the odds separately for each

23. W O R K T O G E T H E R • Hemophilia is an X-linked trait. If a man who is normal for the blood-clotting protein marries a woman who is a carrier, what are the odds that a daughter of theirs will have hemophilia? What are the odds that a son will have this disorder? • What would the phenotypes of the parents have to be to produce a daughter with hemophilia?

24. W O R K T O G E T H E R • Adeline is red-green color blind. Her husband, Gaston, has normal vision. They’ve come to you, a genetic counselor, to find out what the chances are that any of their children will inherit Adeline’s red-green color blindness. “The chances for girls will be different from the chances that boys will inherit your red-green color blindness,” you say. They look puzzled and ask why that is? Create a Punnet square to help them understand.

25. Odin – Orange Male Licorice – Black Male In cats, one coat color gene on the X chromosome has two alleles: orange and black. Sprocket – Calico (orange and black) Female

26. W O R K T O G E T H E R • Cats also have X and Y chromosomes. In cats, one coat color gene is on the X chromosome. One allele for this gene produces black fur. The other allele produces orange fur. Use this to explain why male cats can be orange OR black, but normally only females can be calico or tortoise shell (orange AND black).

27. In humans, whose gamete determines the (genetic) sex of the child? • Mom’s • Dad’s • Equal odds that it could be either • No one knows

28. Duschenne’s muscular dystrophy (DMD) is a devastating neurological disorder that often causes death before the teen years. It is caused by a recessive allele carried on the X chromosome. Is it likely that a girl could be born with this disorder? • Yes, if her mother is a carrier. • Yes, if both parents are carriers. • No, she’d have to have a father with DMD, which is nearly impossible. • No, girls never show X-linked traits.

29. Multiple Alleles • Human blood type (A, B, AB, and O) is determined by a gene that has three alleles. • A and B are co-dominant • O is recessive to both • Though there are three alleles, each person still has only two copies of the ABO gene.

30. A and B alleles produce A and B proteins on the surface of red blood cells. The O allele produces neither of these proteins.

31. W O R K T O G E T H E R • Edmund has type B blood. His father had type AB, while his mother had type O. His wife, Francesca, has type A blood. Both of her parents had type AB. What possible blood types can Edmund and Francesca’s children have?

32. W O R K T O G E T H E R • Barney has type AB blood. Betty has type A blood. What blood types can their children have? What types can they NOT have? • Bernadine has type B blood. Her husband, Rupert, has type A blood. Their daughter, Opal, has type O blood. Assuming no infidelities, how did this happen?

33. W O R K T O G E T H E R • Orson and his wife, Laurita, just had a child. But Orson is not pleased. He has his suspicions about his neighbor, Horatio, and suspects Horatio and Laurita have been a little too friendly. He orders blood tests done and finds that he has Type A blood. Laurita is also Type A. But the baby is Type O. Horatio gallantly offers to have his blood tested, and finds he has Type AB. Who can and who cannot be the father of this child?

34. Genevieve had a blood test done to find out her blood type and learned that she inherited an A allele from her father and a B allele from her mother. What is her blood type? • A • B • AB • O

35. Agnes decides to give blood for the first time. She’s looking forward to getting her blood donor card so that she can find out what her blood type is. “Both of my parents are type AB,” she says, “so I know my blood type can’t be: • A • B • AB • O

36. Multiple Genes • Most human traits are the result of multiple genes. • In some cases (such as skin and hair color), there are multiple copies of the same gene (such as the melanin gene). • In many others, there are many different genes controlling a trait, and the environment may affect how a trait is expressed. (Example: human height)

37. Human skin color is controlled by at least three melanin-producing genes, which are incompletely dominant. eggs sperm This massive Punnet square shows a cross between two people who are heterozygous for all three genes.

38. Chromosomal Abnormalities • Chromosomal abnormalities include: • nondisjunctions (failure of chromatids to separate during meiosis) • deletions of parts of chromosomes • Most chromosomal abnormalities cause cell death, but a few are survivable.

39. Cri-du-chat syndrome is caused by a deletion of a large segment of chromosome 5. Cri-du-chat children often have small head circumference and are severely cognitively challenged. Some may have heart defects, muscular or skeletal problems, or vision problems.

40. Trisomy 21 results in Down Syndrome. Using what you know about meiosis, explain how a fertilized human egg cell can end up with three copies of chromosome 21. If a person with Down Syndrome planned to have a child, could the child inherit Down Syndrome?

41. Nondisjunction of the sex chromosomes is more often survivable than nondisjunctions of somatic chromosomes. As long as the fetus has at least one X chromosome, it can survive.

42. Klinefelter syndrome produces an XXY male. At puberty, Klinefelter males fail to fully develop secondary sex characteristics. Men with this syndrome may or may not be sterile. They have a tendency to gain weight easily and their muscle mass is underdeveloped, but mental function is usually normal.

43. Turner syndrome occurs when a girl inherits only one X chromosome. Turner children are often short, and may show swelling in the hands and feet. Some have heart defects, but most are cognitively normal. Hormone therapy at puberty can help Turner girls grow to normal height and develop secondary sex characteristics.

44. W O R K T O G E T H E R • Use what you know about meiosis and the sex chromosomes to explain: • how a boy could be born with two X chromosomes. • how a girl could be born with only one X chromosome. • why there are no Turner boys.

45. Recap • Traits inherited by classic Mendelian genetics are by far in the minority. • Incomplete dominance, sex linkage, and multiple alleles involve single-gene traits that show unique patterns of inheritance. • Most traits involve multiple genes and gene-environment interactions.