BIO 181- Chapter 13: Modern Understanding of Inheritance

What are the two types of sex chromosomes in humans?

X and Y chromosomes.

Which sex chromosome is larger?

The X chromosome.

Which sex chromosome is smaller?

The Y chromosome

Are the X and Y chromosomes completely homologous?

No. Only small regions at their ends are homologous

What is the SRY gene?

A gene on the Y chromosome required for development of the testes.

What sex chromosomes do typical human females have?

XX.

What sex chromosomes do typical human males have?

XY.

Which sex chromosome is found in every human egg?

An X chromosome

Which sex chromosomes may be found in sperm?

Either X or Y

Which parent determines whether an offspring receives an X or Y chromosome?

The father, because sperm may carry X or Y.

What is a sex-linked gene?

A gene located on a sex chromosome.

What is an X-linked gene?

A gene located on the X chromosome.

How many copies of a recessive X-linked allele does a female usually need to express it?

Two copies

How many copies of a recessive X-linked allele does a male need to express it?

Only one copy.

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

Males have only one X chromosome, so one recessive allele can be expressed

What is meant by a male being hemizygous for X-linked genes?

He has only one copy of most genes on the X chromosome

What are two examples of X-linked recessive disorders from the notes?

Color blindness and hemophilia

What is nondisjunction?

Failure of chromosomes to separate properly during meiosis.

What can happen to gametes after nondisjunction?

One may receive an extra chromosome while another receives none

What is aneuploidy?

An abnormal number of a particular chromosome

How can aneuploid offspring result?

Through fertilization involving a gamete produced by nondisjunction

What is monosomy?

Having only one copy of a particular chromosome

What does monosomy mean numerically?

2n − 1

What is trisomy?

Having three copies of a particular chromosome

What does trisomy mean numerically?

2n + 1

What chromosome abnormality causes Down syndrome?

Trisomy 21.

What does trisomy 21 mean?

There are three copies of chromosome 21.

What can cause abnormal numbers of sex chromosomes?

Nondisjunction of the sex chromosomes.

What chromosome combination occurs in Klinefelter syndrome?

XXY.

Klinefelter syndrome occurs in which sex?

Males.

What is trisomy X?

A condition in females with three X chromosomes: XXX

According to your notes, what is commonly seen in females with trisomy X?

Few unusual physical features; they may be slightly taller than average

What chromosome combination occurs in Turner syndrome?

XO

What is another name for the chromosome condition in Turner syndrome?

Monosomy X.

According to your notes, what reproductive effect occurs in Turner syndrome?

Females are typically sterile

What is the only viable complete monosomy in humans noted in your class notes?

Monosomy X

What is polyploidy?

Having more than two complete sets of chromosomes.

What is triploidy?

Having three complete chromosome sets.

How is triploidy represented?

3n.

What is tetraploidy?

Having four complete chromosome sets

How is tetraploidy represented?

4n

How is polyploidy different from aneuploidy?

Polyploidy changes whole chromosome sets; aneuploidy changes individual chromosome numbers.

What happens to one X chromosome in female mammalian cells?

One X chromosome is randomly inactivated.

When does X-inactivation occur?

During embryonic development.

What happens to an inactive X chromosome?

It condenses into a Barr body.

What is a Barr body?

A condensed, inactive X chromosome.

Why can a heterozygous female be a mosaic for an X-linked character?

Different cells may inactivate different X chromosomes.

What is a deletion?

Loss of a chromosome segment

What is a duplication?

Repetition of a chromosome segment.

What is an inversion?

Reversal of a chromosome segment within the chromosome

What is a translocation?

Movement of a chromosome segment to another chromosome.

What can cause deletion, duplication, inversion, or translocation?

Breakage and rearrangement of chromosomes.

Monosomy vs. trisomy?

Monosomy = one chromosome missing; trisomy = one extra chromosome.

Aneuploidy vs. polyploidy?

Aneuploidy affects individual chromosomes; polyploidy affects entire chromosome sets.

Deletion vs. duplication?

Deletion removes a segment; duplication repeats a segment.

Inversion vs. translocation?

Inversion reverses a segment; translocation moves it to another chromosome.

Why does a recessive X-linked allele affect males more easily than females?

A male has only one X, while a female usually needs the allele on both X chromosomes

What major process in this chapter can produce aneuploidy?

Nondisjunction during meiosis