BIOA01 TERM TEST 2 -- BLOCK 4

Alleles separate during gamete formation (meiosis) so that each gamete carries only one allele for each gene.

What is Mendel's Law of Segregation (First Law)?

Alleles of different genes assort independently into gametes; it only applies if genes are on different chromosomes or far apart on the same chromosome.

What is Mendel's Law of Independent Assortment (Second Law), and what is its main exception?

The probability of two or more independent events happening together is calculated by multiplying their individual probabilities.

What does the multiplication rule (AND rule) state in genetic probability?

The probability of an event that can occur in mutually exclusive ways is calculated by adding their individual probabilities.

What does the addition rule (OR rule) state in genetic probability?

Squares represent males, and circles represent females.

What do squares and circles represent in a standard pedigree chart?

The trait often skips generations, and two unaffected carrier parents can have an affected child.

What is a key pedigree clue for an autosomal recessive trait?

Males have only one X chromosome (hemizygous), so a single recessive mutant allele is enough for them to express the trait, whereas females need two copies.

Why are X-linked recessive traits much more common in human males than females?

A failure of homologous chromosomes or sister chromatids to separate properly during meiosis, resulting in gametes with abnormal chromosome numbers.

What is nondisjunction?

Down syndrome (Trisomy 21), Klinefelter syndrome (XXY), and Turner syndrome (X0).

What are three chromosomal disorders caused by nondisjunction in humans?

He has only one X chromosome, meaning he only carries a single allele for that gene and cannot be heterozygous.

What does it mean when a human male is described as "hemizygous" for an X-linked gene?

1/4 (calculated using the multiplication rule: 1/2 probability of inheriting 'a' from mom AND 1/2 probability of inheriting 'a' from dad).

In a cross between two heterozygous individuals for a standard autosomal gene (Aa x Aa), what is the probability that their next child will be homozygous recessive (aa)?

1/64 (1/4 affected chance per child multiplied three times: 1/4 x 1/4 x 1/4).

A couple, both phenotypically normal carriers of an autosomal recessive disease, plan to have three children. What is the probability that all three children will be affected?

100% of their daughters will be carriers (heterozygous), and 0% of their sons will be affected (sons get their only X from mom).

An affected male with an X-linked recessive disorder has children with a homozygous normal (unaffected) female. What percentage of their daughters will be carriers, and what percentage of their sons will be affected?

Autosomal recessive (if it were X-linked recessive, an affected daughter would have to inherit a mutant X from an affected father).

In a pedigree, two unaffected parents have an affected daughter. What is the exact mode of inheritance for this trait?

1/36 (calculated using the multiplication rule: 1/6 x 1/6).

If you roll two standard six-sided dice, what is the probability of rolling a 6 on the first die AND a 6 on the second die?

4/6 or 2/3 (Even: 2,4,6 [3/6]; >4: 5,6 [2/6]; overlap '6' counted once using addition rule: 3/6 + 2/6 - 1/6 intersection = 4/6).

If you roll a single six-sided die, what is the probability of rolling either an even number (2, 4, 6) OR a number greater than 4 (5, 6)?

1/2 (sons inherit their single X from mom; she has a 50% chance of passing down the X chromosome carrying the colorblindness allele).

A woman who is a carrier for an X-linked recessive colorblindness allele has children with a normal-vision male. What proportion of their sons are expected to be colorblind?

Two gametes will be disomic (n+1, having 24 chromosomes), and two will be nullisomic (n-1, having 22 chromosomes).

During meiosis, a nondisjunction event occurs during Anaphase I in a human oocyte. What are the chromosome number abnormalities in the resulting gametes?

Because the genes are linked; they tend to be inherited together as a block during meiosis unless separated by crossing over.

A gene for eye color and a gene for wing shape in fruit flies are located very close to each other on the same chromosome. When parents are testcrossed, offspring do not follow expected independent assortment ratios. Why?

Because fathers always pass their single X chromosome to all of their daughters and their Y chromosome to all of their sons.

Why can an affected father with an X-linked dominant condition pass the trait to 100% of his daughters but 0% of his sons?