Chapter 12 Human Genetics
1. Explain the role of sex chromosomes in sex determination.
A. The X and Y chromosomes are sex chromosomes
1) Sex chromosomes determine an individual’s sex
a) females have two x chromosomes = XX
b) males have one x chromosome and one y chromosome = XY
2) All of the other chromosomes, those not involved in sex determination, are called
autosomes
3) The X chromosome is larger and therefore carries more genetic information than
the Y chromosome
2. Describe how sex linkage affects the inheritance of traits.
A. The presence of a gene on a sex chromosome is called sex linkage
1) Genes found on the X chromosome are said to be X-linked genes
a) Since males have only one X chromosome, they often inherit and express
recessive X-linked traits such as color blindness
b) Since females have two X chromosomes, they must inherit two recessive alleles
to express the trait of color blindness
2) Genes found on the Y chromosome are Y-linked genes
3. Explain the effect of crossing-over on the inheritance of genes in linkage groups and
summarize the procedure involved in constructing a chromosome map.
A. Linkage groups
1) Genes located on one chromosome form a linkage group
a. 2 or more genes that are found on the same chromosome are said to be
linked
i. Dihybrid crosses
2) The likelihood that crossing-over will result in the separation of 2 genes depends
on how far apart the 2 genes are on the chromosome
a. The farther apart they are, the more likely they are to separate
i. Scientists look at how often genes for particular traits are separated
from one another among offspring - This is useful for constructing
chromosome maps
1. Chromosome maps – a diagram that shows the linear sequence
of genes on a chromosome (i.e. how far away everything is
from one another)
2. These distances are measured in map units
a. Map unit – 2 genes that are separated by crossing over
1% of the time
b. This is useful to see the likelihood that an individual will
inherit certain traits
4. Distinguish between chromosome mutations and gene mutations.
A. Mutations:
1) Germ cell mutations – mutations that affect the germ cells (gametes)
a. These don’t affect the individual but rather their offspring
2) Somatic mutations – mutations that affect the somatic (body) cells
a. These affect the individual, not the offspring.
i. Examples: Cancer
3) Lethal mutations – cause death, often before birth
4) Chromosome mutations – inversion, deletion, duplication, translocation, and
nondisjunction
5) Gene Mutations – involve segments of DNA, both small and large - These generally
affect the protein made
a. Examples: substitution, addition, and deletion - All of these are called point
mutations
i. Additions and deletions result in frame shift mutations (the DNA
sequence is read wrong).
5. Show how pedigree analysis can be used to illustrate the inheritance of traits.
A. Biologists discover how traits are inherited by studying phenotypes among family
members of the same species from one generation to the next
B. In such studies, geneticists often prepare a pedigree, a family record that shows how a
trait is inherited over several generations
C. When analyzing pedigrees, biologists find that certain phenotypes are usually repeated
in predictable patterns from one generation to the next
1) These patterns are called patterns of inheritance
2) Individuals who have one copy of a recessive autosomal allele are called carriers
D. Genetic disorder – disease or conditions that have a genetic basis.
1) Some are caused by single alleles (both dominant and recessive), some are Xlinked, some are sex-influenced, and some traits are polygenic
2) Recently scientists have discovered ways of detecting certain diseases - They do
so using genetic markers
a. Genetic Marker – a short section of DNA that is known to have a close
association with a particular gene located nearby
E. Detecting genetic disorders
1) A person with a family history of a genetic disorder may wish to undergo
genetic screening
a. Genetic screening – an examination of a person’s genetic makeup
i. You can see how likely it is that you will pass on a disorder to your
offspring
ii. If you are found to be a carrier, a doctor may recommend genetic
counseling
(1) Here, doctors and patients discuss the situation and talk about
options
b. Genetic screening can be done in utero via an amniocentesis
i. This is done using chorionic villi sampling
(1) The doctor obtains a piece of the chorionic villi, the tissue that
grows between the mother’s uterus and the placenta (usually
around the 8th to 10th week of pregnancy) - A karyotype of the
baby’s DNA can be constructed to check for abnormalities
c. Sometimes, genetic screening is done immediately after birth
i. Example: checking for phenylketonuria
(1) Phenylketonuria – a genetic disorder in which the body can’t
metabolize the amino acid phenylalanine
(a) When phenylalanine builds up, it causes brain damage
F. Key to pedigrees
1) Squares are males
2) circles are females
3) Darkened squares and circles indicates an affected individual
4) clear squares and circles indicates an individual who is not affected and is not a
carrier
5) Squares and circles that are half filled indicate a carrier that is not affected
4. Explain the inheritance of ABO blood groups.
A. Trait Controlled by Multiple Alleles
1) Multiple-allele traits are controlled by three or more alleles of the same gene that code
for a single trait
2) In humans, the ABO blood groups are controlled by the three alleles A, B, and O.
Each individual’s ABO blood group genotype consists of two of these alleles, which
determine his or her ABO blood type
3) The alleles A and B are codominant (both are expressed when together), and both are
dominant to the O allele
a) a person with type A blood could be either AA or AO
b) a person with type B blood could be either BB or BO
c) a person with type AB blood carries both the A and B alleles
d) a person with type O blood carries two copies of the O allele
Chapter 12 Inheritance Patterns and Human Genetics
 Sex chromosomes determine the sex of an organism.
 A sex-linked gene is found on one sex chromosome but not on the other. Genes found on the
X chromosome are X-linked genes. Genes found on the Y chromosome are Y-linked genes.
 The genes found on one chromosome make up a linkage group. The genes found on a
chromosome are said to be linked.
 Linked genes can be separated by crossing-over. The percentage of crossing-over between
two genes is directly proportional to the distance between them.
 The frequency of crossing-over between genes can be used to construct a chromosome map.
 Crossing-over creates new combinations of genes and therefore serves as a source of genetic
variation.
 Germ-cell mutations occur in gametes and can be passed on to offspring. Somatic mutations
occur in body cells and affect only the individual organism.
 Chromosome mutations are changes in the structure of a chromosome or the loss of an entire
chromosome. Gene mutations are changes in one or more of the nucleotides in a gene.
 A pedigree is a family record that shows how a trait is inherited over several generations.
 Single-allele traits are controlled by a single allele of a gene. Multiple-allele traits are
controlled by three or more alleles of a gene.
 Polygenic traits are controlled by two or more different genes.
 The genes for X-linked human traits, such as colorblindness, Duchenne muscular dystrophy,
and hemophilia, are found on the X chromosome.
 A sex-influenced trait is expressed differently in men than it is in women, even when
individuals have the same genotype.
 Nondisjunction during meiosis can cause gametes to have one too few or one too many
chromosomes. If such a gamete is fertilized, a genetic disorder such as monosomy (2n=45)
or trisomy (2n=47) can occur.
 Genetic screening examines a person’s genetic makeup and can identify people at risk of
passing on genetic disorders to their children. Genetic counseling informs these individuals
about problems that could affect their prospective offspring.
 A karyotype can be used to reveal chromosomal abnormalities.
 Amniocentesis and chorionic villi sampling enable physicians to test a fetus for the presence
of genetic disorders.
Vocabulary List
Carrier
Deletion
Frame shift mutation
Genetic disorder
Genetic screening
Germ-cell mutation
Inversion
Lethal mutation
Linkage group
Map Unit
Monosomy
Multiple-allele trait
Nondisjunction
Pattern of inheritance
Pedigree
Point mutation
Polygenic trait
Sex linkage
Sex-influenced trait
Single-allele trait
Somatic mutation
Substitution
Translocation
Trisomy
X-linked gene
Y-linked gene