AP Biology Heredity Test Review Guide
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
No matter how many children you have, the probability of giving birth to a specific gender each time is
50/50…even if you have 13 boys, and you’re pregnant again!
Testcross always involves a homozygous recessive parent, in an attempt to discover the dominant genotype
of the other parent. (heterozygous, or homozygous dominant)
When a trait is incompletely dominant, as is the case with sheep eye-color (brown-eyed sheep contain the B1
allele, and are homozygous; Green-eyed sheep contain the B allele, and are also homozygous; when mated
together, they produce blue-eyed sheep with a genotype of B1B) When the heterozygous blue-eyed sheep are
mated, you get a 1:2:1 ratio of Brown: Blue: Green
Know how to separate alleles in a dihybrid cross. The parent AaBb would separate as follows: AB; Ab; aB;
ab
Color blindness is sex linked in humans. The ONLY way a son can be color blind is if the mother is at least a
carrier of the condition, as his X chromosome can only come from her.
Identical alleles are homozygous; Different alleles are heterozygous
Hemophilia is a sex-linked trait. If a daughter inherits the disease, her father must have the condition (on his
one and only X chromosome, and her mother must at least be a
carrier, heterozygous)
Using a karyotype (see right), you can determine both gender, and
chromosomal abnormalities, such as Down Syndrome (trisomy
21…or 3 21st chromosomes) This is a female
Know how to determine probability of outcome of crosses,
Such as AaBbcc X aabbCc
What is the chance of producing aaBbcc?
Take the first set of alleles (Aa and aa) and determine the
probability of producing aa. Chances are ½. Now take the second
set of alleles (Bb and bb). Determine the chances of producing Bb.
Chances are ½. Now take the last set of alleles (cc and Cc).
Determine the chances of producing cc from that cross. Chances
are ½. Now, multiply all three fractions together.
½ x ½ x ½ = 1/8. So the chances of the outcome aaBbcc, is 1/8.
White eyed fruit flies are a sex-linked variety, carried on the X chromosome. When red-eyed
homozygous dominant females are crossed with hemizygous recessive white-eyed males, 100% of their
offspring will have red eyes. Genotypes XWXW x XwY = 2XWXw, and 2 XWY
When homozygous recessive females (XwXw) are crossed with hemizygous dominant males
(XWY), the females will all have red eyes, and the males will have white eyes. Use the Punnett
Squares to help make your crosses. While the trait is a sex-linked trait, the real reason for the actual
phenotypic expression of eye color is one of dominance.
Study the pedigree below and to the right. Determine if it is autosomal dominant, or recessive, or
sex-linked. Then predict the possible genotypes of the individuals within the predigree.
If a child with type A blood is born to a mother with type B
blood, what can you determine about the father’s blood
type? Eliminate the blood types that the father cannot be, if
there are any. Father cannot be type B, or type O
If father has type AB, mother has A, and child has B, what
are the probable genotypes of mother and child? Mother is
heterozygous for A (IAi), child is heterozygous for B (IBi).
If a trait, such as achondroplastic dwarfism, is an
autosomal dominant trait that in the homozygous condition
causes death (is lethal), and only shows itself in the
heterozygous condition, what are the chances of an
unaffected male, and a female with the disorder producing
normal children? 1/2
Mendel’s Law of independent assortment is best illustrated
with the dihybrid cross.
Epistasis is the phenomenon where the effects of one gene are modified by one or several other genes, which
are sometimes called modifier genes. Epistasis is often studied in relation to traits which are inherited through
polygenic inheritance, such as coat color in mice which is determined by genes at two loci.