Chapter 12: Human Genetics
Standards and Objectives
A. Identify traits that can and cannot be inherited and be able to distinguish between patterns of
inheritance. (NM – II.II.II.4). 321-322.
1. Hemophilia is a sex linked trait carried on the X chromosome. Females need 2 alleles
to show the disorder, while males only require 1. Interpret the pedigree below:
a. What do the following symbols represent: A square, a circle, an unshaded
circle, a shaded square, a horizontal line and a vertical line.
b. Which individuals are hemophiliacs?
c. Which individuals are carriers?
d. If individual 2 were a female, what is the probability that she would be a
carrier?
2. Simple recessive heredity: a pattern of inheritance that requires 2 recessive alleles in
order to be expressed. An example is Cystic fibrosis in humans (pp. 311-312). If
N=normal, and n= cystic fibrosis, determine the probability of a child having the
disease if both parents are carriers heterozygous) for the disease.
3. Simple dominant heredity: a pattern of inheritance that requires only 1 dominant
gene to be expressed. An example in humans is hitchhiker’s thumb or Huntington’s
disease (pp. 313-314). Analyze the pedigree shown on pg. 314.
a. Which individuals show the disease?
b. Explain why (or how) individuals II. 2 and 4 show the disease, while their
brother does not have the disease.
4. Incomplete dominance: a pattern of inheritance where the heterozygous condition
shows an intermediate phenotype between the two parents. An example is flower color
in snapdragons (p 316). For instance RR= red, R’R’= white, but RR’= pink.
a. What are the genotypic and phenotypic results from a cross of two pink
flowered parents?
5. Codominance a pattern if inheritance where both alleles are expressed. An example
is feather color in chickens- If a bird has the genotype BB, it has black feathers. If a
bird is WW, it has white feathers. If a bird is BW, however, it has both black and
white feathers (pp. 316-317). In humans, sickle cell anemia (p.323-324) shows the
codominant pattern of inheritance:
AA= normal (normal hemoglobin
SS= sickle cell (misshapen hemoglobin)
AS = intermediate or mild sickle cell (both normal and sickle cells- malaria resistant)
a. If a woman who is homozygous normal marries a man who shows mild sickle
cell disease, what are the chances that a child born will be normal? What are
the chances that a child born will have the disease?
6. Polygenic inheritance: a type of inheritance where 2 or more genes, each with
different alleles govern a trait. Examples include cob length in corn and skin color in
humans.
7. Multiple Alleles:. Traits controlled by only one gene, but with more than 2 alleles
(pp. 317, 324-325). In humans the ABO blood group is controlled by multiple alleles.
a. List the possible phenotypes and genotypes for human blood (Notes and p. 325).
b. A man is accused of fathering 2 children, one with type o blood and another with type
A blood. The mother of the children has type B blood and the man has type AB blood.
Could he be the father of both children? Explain your answer.
c. Review your notes on ABO blood compatibility. Your friend, with type A blood,
needs transfusion. You have type AB blood. Can you give your friend blood? Which
blood types are compatible? (Don’t take into account the Rh factor).
B. Know that most cells in the human body contain 23 pairs of chromosomes including one pair
that determines sex, and that human females have two X chromosomes and human males have
an X and a Y chromosome (NM – II.II.II.7). 318-319, 326-329, 334, 335
8. Sex-linked inheritance is a pattern of inheritance where the alleles are carried on the sex
chromosomes (usually the X). In humans hemophilia (blood won’t clot) and red-green
color blindness are sex-linked traits.
XHXH= normal female
XHY=normal male
H h
X X = normal female
XhY=hemophiliac male
XhXh= hemophiliac female
a. What would the genotypes of parents have to be for them to have a
hemophiliac daughter. Explain.
b. The gene for colorblindness follows the same sex linked pattern as
hemophilia with C=normal and c= colorblind. If a man carries the normal
phenotype, is it possible for him to have a colorblind child? Explain.
9. Be able to interpret a karyotype, including the sex of the individual, and any abnormalities
such as a trisomy or a monosomy.
a. Define non-disjunction, monosomy and trisomy.
b. Distinguish between the terms sex-chromosomes and autosomes.
C. Read Chapter 12. Use objectives p. 309, 315 & 323 to focus reading.
D. Define Vocabulary terms p. 333 (11 words)
E. Extra Credit: HW- p. 334 # 1-12. Lab- pp. 334-335 # 13-18. Test EC will be on test.