advanced genetics

Where are genes located?

Genes are located on chromosomes.

What is a gene’s specific location on a chromosome called?

A locus.

What are wild type phenotypes?

Normal traits that are most common in a population.

What are mutant phenotypes?

Traits that differ from the wild type.

How does chromosome behavior during meiosis explain the law of segregation?

Homologous chromosomes separate during meiosis, so each gamete receives only one allele for each gene.

How does chromosome behavior during meiosis explain the law of independent assortment?

Different chromosome pairs align independently during meiosis, so genes on different chromosomes are inherited independently.

The ______ chromosome is larger, and the ______ chromosome is smaller. Only the ______ of the Y chromosome have regions that are homologous with the X chromosome.

• x, y

• ends

During meiosis in males, the ______ and ______ chromosomes separate so that each ______ receives only one sex chromosome. In females, all ______ receive an ______ chromosome.

X; Y; sperm; eggs; X

A gene located on either sex chromosome is called a ______ gene.

sex-linked

What is a sex-linked gene?

A sex-linked gene is a gene located on either sex chromosome. These genes can code for traits unrelated to sex and follow specific inheritance patterns because males and females have different combinations of X and Y chromosomes.

In humans, which chromosome is usually referred to when we say “sex-linked,” and why?

In humans, “sex-linked” usually refers to genes on the X chromosome, because the X is larger and carries many more genes than the Y. This makes X-linked traits much more common and observable.

Why are recessive X-linked disorders more common in males than in females?

Males have only one X chromosome, so a single recessive allele on it will cause the trait to be expressed. Females have two X chromosomes, so a recessive allele on one X can be masked by a normal allele on the other, making females carriers rather than affected.

What is a carrier in terms of X-linked traits?

A carrier is a female who has one recessive allele for a trait on one X chromosome and a normal allele on the other. She does not express the trait but can pass the recessive allele to her children.

How many copies of a recessive X-linked allele are needed for the trait to be expressed in females and males?

A female needs two copies of the recessive allele (one on each X) for the trait to appear, while a male only needs one copy (on his single X) because there is no second X to mask it.

Each X chromosome carries ______ allele(s) for each gene. In females, there are ______ X chromosomes, but only ______ X is active per cell, while the inactive X forms a ______. Males have only ______ X chromosome, so their single allele is always expressed.

one; two; one; Barr body; one