Chapter 12: Mendel and Heredity Study Guide
(Pages 280 – 284)
A. Using a Pedigree
1. What is a pedigree and how is it used?
2. Define the term genetic disorder.
3. Why would a couple want to have a pedigree done?
4. What does it mean when someone is a carrier?
5. Define the term albinism.
6. What causes someone to have albinism?
7. Is albinism caused by a dominant or a recessive allele?
8. What 3 aspects of inheritance can a pedigree help identify (what can a pedigree tell
you?
B. Sex-Linked Genes
1. Identify the 2 sex chromosomes. ________ and ________
2. What is a sex-linked gene?
3. Which sex chromosome holds fewer genes? ______ Why?
4. XNXn – represents a female with a dominant (N) allele on one of her X sex
chromosomes a ______________________ (n) allele on the other X sex
chromosome. This makes her a heterozygous carrier for the recessive trait; she
does NOT express the recessive allele in her phenotype.
5. If a male inherits the Xn chromosome from his mom & the Y chromosome from his
dad, the male will inherit the disorder; there is no dominant allele on the Y sex
chromosome to “mask” the recessive allele.
6. Two sex-linked genetic disorders in humans are
a. __________________________- genetic disorder in which a person cannot
distinguish between 2 colors such as red and green.
b. Hemophilia – genetic disorder in which a person’s _________________ does NOT
clot properly; a serious injury may cause them to bleed to death.
7. Human males inherit the recessive allele for colorblindness and hemophilia from
their __________________, who gives them their X sex chromosome. Females
don’t usually inherit these diseases because they inherit two X sex chromosomes; as
the dominant allele on one of the X sex chromosomes “________________” the
recessive disease-causing allele.
C. Many Genes, Many Alleles – not all characteristics follow Mendel’s pattern
of inheritance known as complete dominance; other patterns of
inheritance include polygenic inheritance, incomplete dominance,
multiple alleles, and codominance.
1. ________________________ inheritance – occurs when several genes work
together to produce a particular trait. Examples: eye, skin, hair color and height in
humans.
2. ________________________ dominance – pattern of inheritance where both alleles
produce a third phenotype. Example: Cross a red snapdragon flower (rr) with a
white snapdragon flower (ww) produces a pink snapdragon flower (rw), a third
phenotype. If you cross 2 pink snapdragon flowers rw x
rw will produce some
¼ red flowers (rr), ½ pink flowers (rw), ¼ white flowers (ww).
3. ___________________ _______________ - genes that have 3 or more
possible alleles, but the individual only carries 2 of the alleles. Example in humans is
blood type. Identify the 3 genes for the ABO blood groups:______, ______, ______
a. How many blood types are there? _________
b. Identify the blood types based on their genotypes:
* IAIA or IAi = blood type ______
* IAIB = blood type ______
* IBIB or IBi = blood type ______
* ii = blood type ______
4. _________________________ - occurs when both alleles for the trait are
expressed in the phenotype. Example: The AB blood type in humans; the IA allele
and the IB allele work are each expressed to give the ______ blood type. In animals,
codominance produces Andalusian chickens; cross a black rooster and a white hen
and the offspring have black & white feathers.
5. Explain how codominance & incomplete dominance differ from one another.
D. Genes Affected by the Environment
1. Name 2 environmental conditions that can affect the phenotype of an organism.
2. Explain, in detail, how temperature affects fur color of the Arctic fox. How does this
benefit the fox?
3. Explain, in detail, how human height may be affected by the environment.
E. Genes Linked Within Chromosomes – the chromosomes of organisms may carry
several hundred to several thousand genes. The closer genes are to one another on the
same chromosome the ___________ likely they are to be separated than genes that are
_______ apart. Genes, and the traits they code for, that are close together on the same
chromosome are said to be _______________.