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Single-Gene Inheritance (Learning Objectives)
Review the presence of homologous chromosomes in diploid organisms that
reproduce sexually, the definitions of “karyotype”, autosomes and sex
chromosomes.
Recognize Mendel’s contribution to genetics and the terminology he used.
Understand and define: characteristic, trait, true-breeder, genotype,
phenotype, allele, autosomal dominant and recessive traits, and a
monohybrid cross.
Explain Mendel’s law of allele segregation.
Learn what is meant by a test cross and when it is used.
Explain Mendel’s law of independent assortment for the simultaneous
inheritance or two characters.
Understand and use the Punnett square for determining genotypes and
phenotypes and probability of offspring for autosomal dominant or recessive
traits.
Learn how pedigrees are used to determine the pattern of inheritance and
make genetic predictions.
Understand the significance of dominance and recessiveness of traits for
Medical Genetics
Patterns of Inheritance
Gregor Mendel
- Priest studied variation in plants, patterns of
inheritance in garden peas
- Described the units of inheritance and the
laws of inheritance, how genes pass from
generation to generation
- Used math to explain biological phenomena
Gregor Mendel
Experimented from 1857-1863
Used:
- Controlled plant breeding
- Careful recordkeeping
- Large numbers
- Statistics
Terminology
Character or characteristic: a heritable
feature e.g. flower color
Trait: variant of the character e.g. purple or
white
Mendel focused on characters with two
variants “either-or” traits
Mendel had control over
which plants he crossed
Mendel Studied Transmission of
Seven Traits in the Pea Plant
Figure
4.2
Figure 4.3
True-Breeding Plants
Offspring have the same trait as parent
Examples:
- Round-seeded parents produce all
round-seeded offspring
- Yellow-seeded parents produce all
yellow-seeded offspring
- Short parents produce all short offspring
Mendel started with
True-breeding plants
F1 generation
F2 generation
F2 ratio
Purple flower- dominant trait
White flower- recessive trait
Mendel's Data
Mendel was looking for a model that can
account for the 3:1 ratio that he observed
in the F2 generation
Mendel’s Model
1. An organism inherits two alleles (one
from each parent).
2. One allele is dominant and the other is
recessive
3. Mendel’s two laws:
- Law of Segregation
- Law of Independent Assortment
Mendel's First Law – Segregation
Figure 4.5
A Punnett square
- Represents
particular genes in
gametes and how
they may combine
in offspring.
- Predicts the
results
of a genetic cross
between
individuals
of known
genotype.
Vocabulary used in Genetics
An organism with two identical alleles is
homozygous for that character.
Organisms with two different alleles for a character
is heterozygous for that character.
A description of its genetic makeup is its
genotype.
A description of an organism’s trait is its
phenotype.
Test Cross
Used to determine
the genotype of a
dominant trait
Eye Color
Wild-type human eye color is brown
- Blue and green eyes stemmed from
mutations or SNPs that persisted
The surface of the
back of the iris
contributes to
the intensity of
eye color
Figure 4.8
Inheritance of Some Common Traits
Box, Figure 1
Reading 4.1, Figure 1
Other Common Mendelian Traits
Mendel's Second Law – Independent Assortment
If the two pairs of
alleles segregate
independently of
each other
Gametes:
P generation YR
and yr
F1 generation YR,
Yr, yR, and yr
These
combinations
produce four
distinct
phenotypes in a
9:3:3:1
ratio. 4.9
Figure
Figure 4.12
• Monohybrid cross- inheritance of one
character with one pair of alleles
• Dihybrid cross- inheritance of 2 characters
with two pairs of alleles
Crossing true-breeding plant that have
yellow, round seeds (YYRR) with truebreeding plants that have green, wrinkled
seeds (yyrr).
Mendel's Second Law – Independent Assortment
Figure 4.11
Figure 4.9
Pedigrees
• Symbolic representations of family
relationships and the transmission of
inherited traits
• Help in understand the past and predicting
the future.
Pedigree Analysis
Figure 4.15
Autosomal Recessive Trait
Albinism = Deficiency in melanin production
Figure 4.17
Autosomal Dominant Trait
Polydactyly = Extra fingers and/or toes
Figure 4.16b
Medical Genetics: Dominance and
Recessiveness
• Reflect the characteristics or abundance of a
protein
• Recessive traits have “loss of function”
• Dominant traits have “gain of function”
• Recessive disorders tend to be more severe
• For many autosomal dominant traits, affected
individuals are heterozygous (Aa)
- The homozygous dominant phenotype (AA) is either
lethal or very rare
Autosomal Dominant Traits
Autosomal Recessive Traits
5. More likely to occur in families with consanguinity