Heredity ❮ 115
- D—This is most likely a sex-linked recessive dis-
ease. The father in the first generation does not
have the condition, so his genotype would be
XNY. The original couple has four children, two
boys with the condition, and one girl and one boy
without the condition. The genotype of the boys
with the condition would be XnY. This means
that the original mother’s genotype would be
XNXn—thus she is a carrier. One of the children
who inherited the condition has children with a
woman from a different family, and neither of their
two children displays the condition.However, the
daughter of son A has three children with a man
who is XnY, and she has a daughter and a son
who show the recessive condition and one
normal son. This means that the daughter of son
A is most likely XNXn—another carrier of the
condition. This disease is a condition that is, accord-
ing to the pedigree, more often seen in men, and
passed along to men by the X chromosome from the
mother. However, it is important to note that if a
father who has the X-linked condition has a child
with a female carrier for the condition, that couple
can indeed produce a female with the condition.
❯ Rapid Review
You should be familiar with the following terms:
Character:heritable feature, such as flower color.
Monohybrid cross:cross involving one character (Bb ×Bb)→(3:1 phenotype ratio).
Dihybrid cross: cross involving two different characters (BbRr × BbRr) → (9:3:3:1
phenotype ratio).
Law of segregation:the two alleles for a trait separate during the formation of gametes—one
to each gamete.
Law of independent assortment:inheritance of one trait does not interfere with the inheri-
tance of another trait.
Law of dominance:if two opposite pure-breeding varieties (BB ×bb) are crossed, all off-
spring resemble BB parent.
Intermediate inheritance:heterozygous (Yy) individual shows characteristics unlike either
parent.
- Incomplete dominance:Yy produces an intermediate phenotype between YY and yy
(snapdragons).
- Codominance:both alleles express themselves fully in a Yy individual—(MN blood groups).
Polygenic traits:traits that are affected by more than one gene (eye color, skin color).
Multiple alleles:traits that correspond to more than two alleles (ABO blood type: IA, IB, i).
Epistasis:a gene at one locus alters the phenotypic expression of a gene at another locus
(coat color in mice).
Pleiotropy:a single gene has multiple effects on an organism (sickle cell anemia).
Sex determination:males are XY, females are XX.
Autosomal chromosome:not involved in gender.
Fruit flies:wild-type traits are the normal phenotype; mutant traits are those that are differ-
ent from normal.
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