Chapter 23: Patterns of Gene Inheritance

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Transcript Chapter 23: Patterns of Gene Inheritance

Inheritance
Gregor Mendel
An Austrian monk, who in 1860 developed certain
laws of heredity (rules for passing of traits from one
generation to the next)
Gregor Mendel investigated genetics at the organismal
level. He experimented with pea plants and their
offspring.
Mendel's goal was to understand how traits are passed
from one generation to the next.
What is a trait?
By quantifying the patterns in which traits (your
inherited characteristics) are inherited, Mendel
developed two "laws".
The law of segregation
- states that each individual has two factors
for each trait but can only pass on one
to their kids.
- today we call the traits genes and the
factors alleles. (alternative forms of a gene)
- alleles have the same position on a pair of
homologous chromosomes (chromosomes for
the same traits)
Mendel’s second law: The law of independent
assortment
The alleles segregate (separate)during the
formation of the gametes (egg and sperm)and each
gamete has only one allele from each pair the
parent possesses .
Each chromosome is passed on independent of the
others (you have 23 pairs, 22 autosome and 1 sex
chromosome)
Fertilization gives each new individual two alleles
for each trait again.
Summary
Alleles are alternative forms of a gene located at one
site on a chromosome; alleles determine the traits of
individuals (your genotype and your phenotype).
Chromosomes and their alleles separate and assort
independently when gametes form; this increases
variety among offspring.
Gene loci - located on homologous chromosomes
How do we inherit a single trait?
We want to know the mode of inheritance.
Dominant allele
- it is expressed when present
- it is designated with a capital (uppercase) letter
- an example is W for widow’s peak.
Recessive allele
- it is only expressed in the absence of a dominant
allele.
- it is designated with a lowercase letter indicates
- An example is w for continuous hairline.
Widow’s peak
WW or Ww
ww
Genotype versus Phenotype
Genotype refers to the genes of an individual which
can be represented by two letters or by a short
descriptive phrase.
Homozygous means that both alleles are the same;
for example,
WW stands for homozygous dominant
ww stands for homozygous recessive.
Heterozygous means that the members of the allelic
pair are different
for example, Ww, a heterozygote
Phenotype refers to the physical or observable
characteristics of the individual.
Both WW and Ww result in widow’s peak,
- the phenotype is a widow's peak
- however, we have two genotypes resulting in
the same phenotype.
ww results in a straight hairline
- in this case, the phenotype can only result from
one genotype
Gamete Formation
Because homologous pairs separate during meiosis, a
gamete has only one allele from each pair of alleles.
If the allelic pair is Ww, a gamete would contain either a
W or a w, but not both.
Ww represents the genotype of an individual.
Gametes are represented by W or w.
One-Trait Crosses
In one-trait crosses, only one trait such as type of
hairline is being considered.
When performing crosses, the original parents are
called the parental generation, or the P generation.
All of their children are the filial generation, or F
generation.
In a one-trait cross, if one parent is homozygous dominant
and the other is homozygous recessive, all offspring of the
filial generation with be heterozygous for that trait.
If you know the genotype of the parents, it is possible to
determine the traits of the gametes.
A Punnett square can then be used to determine the
phenotypic ratio among the offspring.
When a heterozygote (monohybrid) reproduces with
another heterozygote (monohybrid), the phenotypic ratio
of the offspring will be 3 expressing the dominant trait to
1 expressing the recessive trait (3:1 ratio).
A 3 : 1 ratio means that there is a 75% chance of the
dominant phenotype and a 25% chance of the recessive
phenotype.
Monohybrid
cross
One-Trait Crosses and Probability
Laws of probability alone can be used to determine
results of a cross.
The laws are:
1) the probability that two or more independent
events will occur together is the product of their
chances occurring separately.
2) the chance that an event that can occur in two or
more independent ways is the sum of the
individual chances.
In the cross of Ww x Ww, what is the chance of
obtaining either a W or a w from a parent?
Chance of W = ½, or chance of w = ½
The probability of these genotypes is:
The chance of WW = ½ x ½ = ¼
The chance of Ww = ½ x ½ = ¼
The chance of wW = ½ x ½ = ¼
The chance of ww = ½ x ½ = ¼
The chance of widow’s peak (WW, Ww, wW) is ¼ + ¼ +
¼ = ¾ or 75%.
The One-Trait Testcross
PROBLEM: It is not always possible to discern a
homozygous dominant from a heterozygous
individual by inspection of phenotype.
A testcross crosses the dominant phenotype with the
recessive phenotype.
If a homozygous recessive phenotype is among the
offspring, the parent must be heterozygous.
One-trait
testcross
Possibility I
Possibility II
The Inheritance of Many Traits
Independent Assortment
The law of independent assortment states that each
pair of alleles segregates independently of the other
pairs and all possible combinations of alleles can
occur in the gametes.
This law is dependent on the random arrangement
of homologous pairs at metaphase.
Segregation and independent assortment
Two-Trait Crosses
In two-trait crosses, genotypes of the parents require four
letters because there is an allelic pair for each trait.
Gametes will contain one letter of each kind in every
possible combination.
Crossing individuals who are heterozygous for two traits
can produces four phenotypes.
The ratio of these four phenotypes will be in a 9:3:3:1.
Two-Trait Crosses and Probability
It is possible to use the two laws of probability to arrive
at a phenotypic ratio for a two-trait cross without using
a Punnett square.
The results for two separate monohybrid crosses are as
follows:
Probability of widow’s peak = ¾
Probability of short fingers = ¾
Probability of straight hairline = ¼
Probability of long fingers = ¼
The probabilities for the dihybrid cross:
Probability of widow’s peak and short fingers
= ¾ x ¾ = 9/16
Probability of widow’s peak and long fingers
= ¾ x ¼ = 3/16
Probability of straight hairline and short fingers
= ¼ x ¾ = 3/16
Probability of straight hairline and long fingers
= ¼ x ¼ = 1/16
Genetic Disorders
Patterns of Inheritance
When studying human disorders, biologists often
construct pedigree charts to show the pattern of
inheritance of a characteristic within a family.
The particular pattern indicates the manner in which a
characteristic is inherited.
Genetic counselors construct pedigree charts to
determine the mode of inheritance of a condition.
Autosomal Recessive Disorders
Tay-Sachs Disease
Tay-Sachs disease is common among United States
Jews of central and eastern European descent.
An affected infant develops neurological impairments
and dies by the age of three or four.
Tay-Sachs results from a lack of hexosaminidase A and
the storage of its substrate in lysosomes.
Cystic Fibrosis
Cystic fibrosis is the most common lethal genetic
disorder among Caucasians.
A chloride ion transport protein is defective in affected
individuals.
Normally when chloride ion passes through a
membrane, water follows.
In cystic fibrosis patients, a reduction in water results in
a thick mucus which accumulates in bronchial
passageways and pancreatic ducts.
Phenylketonuria (PKU)
Individuals with phenylketonuria lack an enzyme needed
for the normal metabolism of phenylalanine, coded by an
allele on chromosome 12.
Newborns are regularly tested for elevated phenylalanine
in the urine.
If the infant is not put on a phenylalanine-restrictive diet
in infancy until age seven when the brain is fully
developed, brain damage and severe mental retardation
result.
Autosomal Dominant Disorders
Neurofibromatosis
Small benign tumors, made up largely of nerve cells,
occur under skin or on various organs.
The effects can range from mild to severe, and some
neurological impairment is possible; this disorder is
variably expressive.
The gene for this trait is on chromosome 17.
Huntington Disease
Individuals with Huntington disease experience
progressive degeneration of the nervous system and no
treatment is presently known.
Most patients appear normal until middle age.
The gene coding for the protein huntingtin contains
many more repeats of glutamines than normal.
Polygenic and Multifactorial Inheritance
Polygenic traits are governed by more than one gene
pair.
Several pairs of genes may be involved in determining
the phenotype.
Multifactorial traits are governed by more than one gene
and environmental factors.
Height is a
multifactorial
trait.
Skin Color
The inheritance of skin color, determined by an
unknown number of gene pairs, is a classic example of
multifactorial inheritance.
A range of phenotypes exist and several possible
phenotypes fall between the two extremes of very dark
and very light.
The distribution of these phenotypes follows a bellshaped curve.
Multifactorial traits and disorders
Many human traits, like allergies, schizophrenia,
hypertension, diabetes, cancers, and cleft lip, appear to
be due to the combined action of many genes plus
environmental influences.
Many behaviors, such as phobias, are also likely due to
the combination of genes and the effects of the
environment.
Multiple Allelic Traits
Inheritance by multiple alleles occurs when more than
two alternative alleles exist for a particular gene
locus.
A person’s blood type is an example of a trait
determined by multiple alleles.
Each individual inherits only two alleles for these
genes.
ABO Blood Types
A person has two alleles. They can be:
- two A antigen (blood type A)
- two B antigen (blood type B)
- one each of A and B antigens (blood type AB)
- or the O antigen (blood type O)
Human blood types can be
type A (IAIA or IA i)
type B (IBIB or IBi)
type AB (IAIB)
or type 0 (ii).
Incompletely Dominant Traits
Codominance means that both alleles are equally
expressed in a heterozygote.
Incomplete dominance is exhibited when the heterozygote
shows not the dominant trait but an intermediate
phenotype, representing a blending of traits.
Such a cross would produce a phenotypic ratio of 1:2:1.
Incomplete
dominance
Sickle-Cell Disease
Sickle-cell disease is an example of a human disorder
controlled by incompletely dominant alleles.
Sickle cell disease involves irregular, sickle shaped red
blood cells caused by abnormal hemoglobin.
HbA represents normal hemoglobin
HbS represents the sickled condition.
HbA/HbA individuals are normal;
HbS/HbS individuals have sickle-cell disease
HbA/HbS individuals have the intermediate condition
called sickle-cell trait.
Heterozygotes have an advantage in malaria-infested
Africa because the pathogen for malaria cannot exist in
their blood cells.
This evolutionary selection accounts for the prevalence of
the allele among African Americans.