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Genetics
Warm-Up
• If all of these puppies came from the
same two parents and were born in the
same litter…why don’t they look identical?
Mendel’s Genetics
• Genetics
• A branch of biology that studies
heredity, or the passing of
characteristics from parents to
offspring
Gregor Mendel
• Austrian monk that first studied genetics
• “Father of Genetics”
• Experimented on pea plants
• Mendel used plants that had been bred
for a specific trait (tallness, shortness) to
create hybrids (heterozygotes)
Generations are named as follows:
• P = original parents
• F1 = first generation (children)
• F2 = second generation (grandchildren)
Mendel’s Conclusions (#1)
•
There are 2 forms of genes that control
each trait called alleles
• Example – 2 genes that control height
• T = tall
• t = short
Mendel’s Conclusions (#2)
• The Rule of Dominance
• Dominant trait = the observed trait
• Recessive trait = hidden by the dominant
trait unless dominant is not present
Mendel’s Conclusions (#3)
• Law of Segregation
• Because each plant has two alleles for a
trait, it can produce two types of gametes
• Pure tall (homozygous) = TT
• Pure short (homozygous) = tt
• Hybrid (heterozygous) = Tt
Mendel’s 3 Principles
1. Principle of Segregation
1. Principle of Dominance
2. Principle of Independent Assortment
Principle of Segregation
• During gamete formation, there is a
random segregation (separation) of
alleles whereby each gamete receives
only one allele from each pair
• Tt
•T
•t
Principle of Dominance
• The dominant allele (capital letter) are
expressed whereas recessive alleles
(lowercase letter) are not because they
are masked by the dominant allele
Principle of Independent Assortment
• Each characteristic is inherited
independently of every other trait
• Dihybrid crosses
STOP
Monohybrid Crosses
• Step 1
• What are the genotypes of the parents?
• Step 2
• Draw the Punnett Square
• Step 3
• Write in the alleles of the parents
• Step 4
• Fill in the square
• Step 5
• Write the genotypes and phenotypes of potential offspring
Concept 10.3
• There are many variations of inheritance
patterns.
• Key Terms
• Intermediate inheritance
• Codominance
• Polygenic inheritance
Intermediate Inheritance
• In a monohybrid cross where neither
parent trait is dominant, the hybrids
have an intermediate phenotype
• Andalusian Chickens
Codominance
• Both traits are expressed
– the phenotype shows
the separate traits of
both alleles
Incomplete Dominance
• A cross between a blue blahblah bird and a white
blahblah bird produces offspring that are silver.
• What are the genotypes of the parent blahblah birds in
the original cross?
• What is/are the genotypes of the silver offspring?
• What would be the phenotypic RATIO of offspring
produced by two silver blahblah birds?
Codominance
• Predict the phenotypic ratios of
offspring when a homozygous red cow
is crossed with a roan bull.
• What should the genotypes and
phenotypes for parent cattle be if a
farmer wanted only cattle with red fur?
• A cross between a black cat and a tan cat
produces a tabby pattern (black and tan fur
together).
• What type of inheritance does this illustrate?
• What percent of kittens would have tan fur if a
tabby cat is cross with a black cat?
Polygenic Inheritance
• 2 or more genes affect a single character
• In humans, height and skin color have
polygenic inheritance
Dihybrid Cross
• In rats, black (B) fur is dominant to
white (b) fur and long (L) tails are
dominant to short (l) tails.
• Cross two heterozygous individuals.
The Importance of Environment
• Siamese cats
• Enzyme responsible
for black fur color is
only active at the
cooler temperatures
Concept 10.4
• Meiosis explains Mendel’s principles.
• Key Terms
• Chromosome theory of inheritance
• Gene locus
• Genetic linkage
Chromosome Theory of Inheritance
• Genes are located
on chromosomes
and the behavior
of chromosomes
during meiosis
and fertilization
accounts for
inheritance
patterns.
Genetic Linkage and Crossing Over
Genetic linkage continues as homologous
chromosomes separate in the formation
of sex cells
Genetic Linkage and Crossing Over
Crossing-over unlinks alleles of genes as
homologous chromosomes separate in
the formation of sex cells
Concept 10.5
• Sex-linked traits have unique inheritance
patterns.
• Key Terms
• Sex-linked genes
Sex-Determination
There are 2 types of chromosomes
1. Autosomes :
Body chromosomes = 22 pairs (44 total)
2. Sex Chromosomes:
23rd pair (X or Y)
Female = XX
Male = XY
Which parent determines the sex of the offspring?
Sex-Linked Inheritance
• Traits controlled by genes located on the sex
chromosome are called sex-linked traits
• Thomas Hunt Morgan discovered sex linkage
of traits using Drosophilia (fruit flies)
Sex-Linked Traits
White eyes in Drosophilia are a recessive trait.
Give the genotypes for the following individuals
•
•
•
•
White-eyed male:
Red-eyed male:
White-eyed female:
Red-eyed female:
Cross a white-eyed male and a
homozygous red-eyed female.
• Offspring Genotype:
• Offspring Phenotype:
Sex-Linked Practice #1
• When crossing a white-eyed male and a
heterozygous red-eyed female - what is
the percentage of male children with
white eyes?
Sex-Linked Practice #2
• Cross a red-eyed male and a whiteeyed female.
• Give offspring genotypes and
phenotypes.
Sex-Linked Practice #3
• Cross a homozygous red-eyed male and
a heterozygous red-eyed female. What
is the percentage that they will produce
a red-eyed male?