Bio Lecture 27

Why can't the term complete dominance be applied to X-linked traits in males?

They only have one X chromosome. Dominance is saying that there are two alleles and one masks the other.

A butterfly inherits the long antenna (A) and plain body (B) alleles from its mother and short antenna (a) and striped body (b) from its dad. List the gamete genotypes..

Different chromosomes: Four different gamete genotypes AB, Ab, aB, ab Independent orientation results in all possible combinations of alleles Same chromosomes: Two different gamete genotypes AB or ab As long as cross over does not occur between genes, alleles on the same chromosome will stay "packaged" together

list the possible genotype(s) for female and male flies with red or white eyes. (red is dominant and white is recessive). List the possible gamete genotype(s) for each fly. XR XR female male Xr Y

Female: X^R X^R or X^R X^r AND X^r X^r Male: X^R Y AND X^r Y XR Xr (females) XR Y (males)

Explain why the gametes produced in males that have X chromosomes always have the same alleles for all X-linked genes. Explain why gametes produced in females can have different alleles.

Males only have ONE allele for each X chromosome gene while females have TWO alleles for each X chromosome gene (alleles can be the same or different)

X-linked recessive diseases are due to a recessive allele for a particular X chromosome gene in humans. Explain why X-linked recessive diseases affect more males than females.

Males only have one X chromosome with one allele

Could you determine the dominant trait for an X-linked gene by only looking at the male offspring?

No- dominant traits refer to phenotypes observed when two different alleles are present. Males only have one allele present for each X linked gene.4

Is there a difference between the number of gamete genotypes that can be produced between chromosomes with genes on on the same chromosome versus different chromosome?

Yes these two numbers are different as long as there is no crossing over involved...

In fruit flies, red eyes is dominant and white eyes is recessive. A red-eyed female fruit fly mates with a white-eyed male. If some of their male offspring have white eyes, what (if anything) do you know about the genotype of the female fly?

The female was most likely heterozygous for red eyes

Red-green color blindness is due to an X-linked recessive allele in humans. A widow's peak (a hairline that comes to a peak in the middle of the forehead) is due to an autosomal dominant allele. A man with normal color vision and a widow's peak marries a color blind woman with a straight hairline. The man's father had a straight hairline, as did both of the woman's parents. Use "W" and "w" for the hairline alleles (widow's peak and straight), and the "XN" and "Xn" for the color vision alleles (normal color vision and red-green color blindness). The genotype of the man with a widow's peak and normal color vision is _________. The genotype of the color-blind woman with a straight hair line is _______. If the couple has a child, what's the probability the child will be a son with a widow's peak? What is the probability of the couple having a color blind child (son or daughter) with a straight hairline? What is the probability of the couple having a color blind daughter with a widow's peak? The couple has a daughter with normal vision and a widow's peak. What is the chance that she is heterozygous for both genes?

Ww XNY; ww XnXn; 25%; 25%; 0%; 100% Mastering Bio Input: Because the genes are on different chromosomes (autosomal chromosome vs. X chromosome), they are inherited independently. The man's father had a straight hairline, so he must have two recessive alleles, which means the man inherited a recessive allele from his father. For the man to have a widow's peak, his other allele must be the widow's peak allele (i.e. he must be heterozygous for the hairline gene). The woman must have two recessive alleles because her hairline is straight. For the color vision gene, the man must have a normal color vision allele because he has normal color vision. The woman must have two recessive alleles to be color blind. The woman's genotype is therefore ww XnXn, and her only possible gamete genotype is w Xn. The man's genotype must be Ww XNY, which means his gametes could be W XN, or w XN, or W Y, or w Y. Each male gamete genotype is equally probable (25%). If you make a Punnett square using those gamete genotypes, you can determine the possible offspring phenotypes and their probabilities (see below). There are four possible offspring genotypes (Ww XNXn, Ww XnY, ww XNXn, wwXnY), and the probability of each is 25%.

A couple has a son and a daughter that are both affected by the same genetic disease. Neither of the parents is affected. From this information, the disease

must be an autosomal recessive disease. Mastering Bio Input: The disease described must be an autosomal recessive disease, and both parents must be carriers. Both children (son and daughter) must have inherited one disease allele from each parent. Since neither of the parents are affected, the disease cannot be an autosomal dominant disease (one of the parents would have been affected if it was an autosomal dominant disease). Females that have X-linked recessive diseases must have TWO copies of the disease allele to be affected - one inherited from their father and one from their mother. Since the father in this scenario was NOT affected, he could not have had a disease allele to pass on to his daughter. Sons inherit a Y chromosome from their father, so sons cannot inherit any X-linked alleles from their father.

Can a male be heterozygous for an X-linked gene? Why or why not?

no because it would require them to have two alleles and they only have one.