Sex Linked Inheritance

Question 1

how many types of sex chromosomes do mammals have (including humans)? what are they?

Answer 1

• two types

- X chromosome and Y chromosome

Question 2

what does the Y chromosome determine in humans and other mammals?

Answer 2

sex

• y present = male
• y absent = female

Question 3

when do X and Y chromosomes pair and then separate?

what does this result in?

Answer 3

pair in meiosis as homologous and separate in anaphase I.

-results in half of sperm having X chromosome, and half of sperm having Y chromosome

Question 4

what is an x-linked gene? what is it also known as?

Answer 4

is a gene located on the x-chromosome

-also known as a sex-linked gene

Question 5

what are 2 features of the X chromosome? what does the X chromosome determine?

Answer 5

• x chromosomes are large and contain many genes

- genes that are not related to sex-determination

Question 6

what are the implications for expressing recessive x chromosome alleles (phenotypes) in females vs males? what does this mean for recessive sex-linked disorders?

Answer 6

females:
-will only express recessive phenotype, if homozygous recessive –> meaning both x chromosome from mother and father are recessive
males:
-will express recessive phenotype, if he gets a recessive allele from his mother

-much more likely for males to have recessive sex-linked disorders than females –> because females would have to have both X chromosomes with the recessive disorder from both mother and father

Question 7

how do you write the genotype for a X-linked gene?

Answer 7

A = dominant allele
a = recessive allele

Female: X^A X^A or X^A X^a or X^a X^a
Male → X^A Y or X^a Y

Question 8

fathers pass X chromosome to…. but not to …

and y chromosomes to … but not to…

Answer 8

pass X chromosome to ALL daughters but to
none of their sons
-pass Y chromosomes to ALL of their sons, but not to any of their daughters.

Question 9

mothers pass X chromosomes to…

Answer 9

both their sons and daughters, meaning it could result in recessive or dominant alleles, depending on mothers genetic makeup

Question 10

how do you determine if a gene is autosomal or

sex-linked?

Answer 10

-preform a reciprocal cross
if Sex-linked inheritance: different phenotypic ratios in reciprocal crosses
if autosomal inheritance: same phenotypic ratios will occur in reciprocal crosses

Question 11

what is a reciprocal cross?

Answer 11

pair of crosses where the phenotypes of the female and male are reversed