Bio 101 Ch. 24

What is crossing over?

Occurs during meiosis between nonsister chromatids in a tetrad. The chromatids exchange genetic material

How many pairs of chromosomes (and total number) do humans have?

23 pairs, 46 total

How many chromosome pairs are autosomes?

22

How many chromosome pairs are sex chromosomes?

1 (X and Y)

Female Sex Chromosome

XX

Male Sex Chromosome

XY

What are sex linked traits?

Traits that are controlled by genes on the sex chromosomes

X-linked recessive disorders and examples

• More males than females will have the disorder

  • recessive allele on the X chromosome is always expressed in males

• Y chromosome lacks an allele for the disorder

EX: colorblindness, fragile X syndrome, Hemophilia

X-linked dominant disorders and examples

• Only a few are known

• Affected males pass the trait only to daughters who have a 100% chance of inheriting the disorder

• Females can pass the dominant allele to both sons and daughters

EX: incontinentia pigmenti

Nondisjunction

A failure of the chromosomes or sister chromatids to separate during cell division—results in trisomy or monosomy when fertilized

Kayotype

Chromosomes arrange by pairs according to their size and general apperance. Chromosomes are numbered and arranged in pairs from the largest to the smallest

What is trisomy 21?

Down Syndrome—most common autosomal trisomy among humans. Easy to recognize (physical features). Diagnosis possible via kayotyping

What is the process of making eggs called?

oogenesis

What is the process of making sperm called?

spermatogensis

Example of Sex Chromosome Disorders

Jacob’s Syndrome: XYY males result from nondisjunction during spermatogensis. Usually taller, suffer from persistant acne and tend to have speech and reading abnormalities. Typically there are no behavioral differences.

Deletions

occurs when a break causes a chromosome to lose an end piece or when 2 simultaneous breaks lead to the loss of an internal chromosome segment

Syndrome examples of chromosome deletions

• William’s Syndrome: children have pixie-like features; poor academic performance but have excellent verbal and musical skills

• Cri du Chat: children are mentally impaired with facial abnormalities, their cries are said to sound like a cat’s

Duplication

Chromosomal segment is repeated in the same chromosome or in a non-homologous chromosome

Inversion

The segment joins in an opposite direction from normal

• Or the segment is rotated 180 degrees

Translocation

The exchange of chromosomal segments between 2 non-homologous chromosomes