GENETICS
Chapter 14
Genetics & Inheritance
Genetics – study of inheritance (heredity), how traits are passed from one generation to the next
Patterns of Inheritance – thoughts in the 1800s: Blending Inheritance Inheritance of Acquired Characteristics
Inheritance Hypotheses (1800s)
Inheritance of acquired characteristics
“pangenes” from organs passed on to homonculus
Jean-Baptiste Lamarck
Homunculus
Gregor Mendel
Austrian monk
Studied genetics using garden pea plants
Mathematician – applied statistics and laws of probability to his experiments
Developed a particulate theory of inheritance
Gregor Mendel – father of modern genetics
Garden PeasFirst Model Organism in Genetics
Easy (and inexpensive) to grow
Produce many offspring
Short reproductive cycle
Able to control matings
Easily recognizable traits
7 different traits
2 options for each trait
Garden PeasControlling Mating
Self-fertilization (self-pollination) – pollen grains (contain sperm) from one flower are used to fertilize the carpel (contains eggs) of the same flower
Carpel (female organ) receives pollen
Eggs
Stamen (male organs) produce pollen grains
Cross-pollination – pollen from the flower of one plant fertilizes the carpel on a flower of a different plant
3. Transfer pollen to the female organs of the individual whose male organs have been removed.
2. Collect pollen from a different individual.
1. Remove male organs from one individual.
?
Garden PeasControlling Mating
Mendel’s Experiments
Started with true-breeding plants; when self-crossed, every generation has same phenotype
Experiment 1:Crossed two true-breeding plants that had alternate phenotypes – recorded phenotypes of offspring (F1)
Experiment 2:Crossed F1 plants with one another – recorded phenotypes of offspring (F2)
Mendel’s Experiments
Experiment #1:
true-breeding purple x true-breeding white
Mendel’s Experiments
Experiment #1: Results
What happened to the “white” trait? Missing? Masked?
Mendel’s Experiments
Experiment #2: A Monohybrid Cross
Self-fertilize F1 generation plants(offspring from Experiment 1)
Mendel’s Experiments
Experiment #2: Results
3:1 ratio
Conclusion: Genetic information for
“white” was only masked
Dominant and Recessive Traits
Genetic information that codes for a particular trait can be:
Dominant – masks the trait of recessive genetic information paired with it (A)
Recessive – trait is fully or partially masked by dominant genetic information (a)
Mendel’s ConclusionsParticulate Inheritance Hypothesis
Parents transmit information about traits to their offspring as hereditary “factors” (particles) hereditary “factors” do not blend together or acquire new
or modified characteristics through use, rather… hereditary “factors” maintain their integrity from
generation to generation
Hereditary “factors” inherited from parents determine traits observed in offspring
Mendel’s Experiments
3:1
See 3:1 ratio with
all of these traits
Mendel’s ConclusionsPrinciple of Segregation
A 3:1 ratio is possible if…
Each parent has two hereditary “factors” for each trait
The factors separate (segregate) during the formation of gametes
Each gamete contains only one factor for a particular trait
Fertilization (random) gives each new individual two factors for each trait
How do we put this into modern genetic terms? Let’s review…
Genetics TerminologyReview
Genes – units of genetic information (DNA) about specific traits; hereditary “factors” Passed from parents to offspring Carry directions for the synthesis of polypeptides/proteins
Alleles – alternate versions of a gene, code for different forms of the same trait
Homologous chromosomes – pair of chromosomes have the same size and shape carry the same genes (alleles may differ)
Gene locus – location of a gene on a chromosome
Genetics TerminologyReview
A pair of homologous chromosomes
A gene locus
Alternate versions of a gene (alleles)
B b
Alleles
What is the source of new alleles?
Mutations – heritable changes in the molecular structure of DNA• Original source of all new alleles• source of diversity in life• alter gene (DNA) alter protein alter trait
Alleles
All individuals have two alleles at each gene locus; one inherited from each parent
The alleles can be the same – homozygous for that gene
The alleles can be different – heterozygous for that gene
A pair of homologous chromosomes
Homozygous (identical alleles)
Heterozygous (different alleles)
B b
A A
Alleles
If the two alleles of a gene differ, the one that is observable is called the dominant allele (A), the one that is masked is called the recessive allele (a)
Dominant – allele that masks the trait of any recessive allele paired with it (A)
Recessive – allele whose trait is fully or partially masked by a dominant allele (a)
Genetics Terminology
Genotype – the set of alleles an individual receives at fertilization
Phenotype – an individual’s physical appearance (observable traits)
genotype determines phenotype
Genotypes and Phenotypes
Possible genotypes & phenotypes for one gene:
Genotypes Phenotypes
Homozygous Recessive
aa Recessive trait
Heterozygous Aa Dominant trait
Homozygous Dominant
AA Dominant trait
Mendel’s ConclusionsParticulate Inheritance Hypothesis - Modified
Parents transmit information about traits to their offspring as particular forms of hereditary “factors”- alleles alleles do not blend together alleles maintain their integrity from generation to
generation
Alleles inherited from parents determine phenotype
Sexual Reproduction
Aa fertilization produces
heterozygous offspring
A A A Aa
aa
a
meiosis I
A A A A a a a a
meiosis II
gametes
AA
AAA
aa
aaaa
chromosomes duplicated before
meiosis
Homozygous dominant parent
Homozygous recessive parent
A
Two alleles for gene A
Genotype: aaPrinciple of Segregation
Each gamete contains only one allele for each geneFertilization gives new
individual two alleles for each gene
(one from each parent)
Punnett Square Analysis
Method used to determine possible genotypes (and phenotypes) of offspring
Steps: Determine the genotypes of all possible gametes
donated by mother (eggs) Determine the genotypes of all possible gametes
donated by father (sperm) Determine all possible combinations of gametes - this
represents all possible genotypes of offspring
Mendel’s ResultsExplained Using the Principle of Segregation
Experiment 1:
true-breeding purple x true-breeding white
What are the genotypes of these
flowers?
Mendel’s ResultsExplained Using the Principle of Segregation
Experiment 1:
homozygous purple x homozygous whitePP x pp
Results: All offspring are purple
Mendel’s ResultsExplained Using the Principle of Segregation
Experiment 1:Parents have two
copies of each gene (diploid)
Gametes have one copy of each gene
(haploid)
Offspring have two copies of each gene
(diploid)
Mendel’s ResultsExplained Using the Principle of Segregation
Experiment 2: Monohybrid Cross• Examine pattern of inheritance of alleles at one gene• Cross individuals that are heterozygous (hybrid) at one
(mono) gene
heterozygous purple x heterozygous purplePp x Pp
Results: 3 purple:1 white
Mendel’s ResultsExplained Using the Principle of Segregation
Experiment 2: Monohybrid Cross
Mendel’s ResultsReview of the Principle of Segregation
Principle of Segregation:
1. Each individual has two alleles of each gene – located on pairs of homologous chromosomes
2. The alleles segregate (separate) during gamete formation (meiosis)
meiosis
meiosis
Mendel’s ResultsReview of the Principle of Segregation
Principle of Segregation:
3. Each gamete contains only one allele for each trait
4. Fertilization gives each new individual two alleles for each gene (one from each parent)
meiosis
meiosis
Mendel’s ResultsReview of the Principle of Segregation
Principle of Segregation:
3. Each gamete contains only one allele for each trait
4. Fertilization gives each new individual two alleles for each gene (one from each parent)
fertilization
Monohybrid Cross
Aa x Aa Phenotypic ratio = 3:1
3/4 dominant phenotype 1/4 recessive phenotype
Genotypic ratio = 1:2:1 1/4 homozygous dominant 2/4 heterozygous 1/4 homozygous recessive
Rr
Rr
R r
R
r
RR Rr
Rr rr
Determining Probability
What is the probability that parents that are heterozygous for the freckles gene will have a child that does NOT have freckles?
F = frecklesf = no freckles
Determining Probability
What is the probability that they will have a child that DOES have freckles?
sum rule – when the same event can occur in more than one way (ex. FF, Ff, and Ff), add results
Determining Probability
What is the probability that they will have two children without freckles?
product rule – chance of two independent events occurring together is the product of their chance of occurring separately
Mendel’s Experiments
How are alleles of different genes (located on different chromosomes transmitted to gametes? Together - dependent assortment or… Separately - independent assortment
T
Tg gt
GG
t
Do the maternal alleles (tg) and paternal alleles (TG) always stay together during segregation?
or…
Do you find a mix of maternal and paternal alleles in the gametes?
Mendel’s Experiments
How are alleles of different genes (located on different chromosomes transmitted to gametes? Together - dependent assortment or… Separately - independent assortment
Experiment: Dihybrid Cross Look at pattern of inheritance of two genes Cross individuals that are heterozygous (hybrid) at
two (di) genes
Generate dihybrid individual by crossing individuals that are homozygous for two traits
Cross dihybrid individuals…
Dihybrid Cross
F1 offspring all TtGg
Dihybrid Individual
Parents have two copies of each gene
(diploid)
Gametes have one copy of each gene
(haploid)
Offspring have two copies of each gene
(diploid)
If Transmission is Dependent…
T and G stay together in gametes;
t and g stay together in gametes
Phenotypic Ratio
3:1
If Transmission is Independent…
Phenotypic Ratio
9:3:3:1
Alleles at T gene and G gene segregate to gametes independently of each other
Dihybrid Cross (Independent Assortment)
AaBb x AaBb Phenotypic ratio = 9:3:3:1 Genotypic ratio = 1:2:1:2:4:2:1:2:1
Mendel’s Principle of Independent Assortment
Genes located on one chromosome are sorted for distribution into gametes independently of genes located on another chromosome
Maternal alleles (tg) do not always stay together;
paternal alleles (TG) do not always stay together
Why is this the case? Think about the steps in meiosis!
T
Tg gt
GG
t
Metaphase I of Meiosis
Metaphase I Independent Assortment – homologous chromosomes line
up at metaphase plate randomly (and will therefore be distributed into daughter nuclei randomly)
Meiosis I
Replicated chromosomesprior to meiosis
Gam
etes
Alleles for seed shape
Meiosis II
Principle of independent assortment: The genes for seed shape and seed colorassort independently, because they are located on different chromosomes.
Meiosis II
Meiosis I
Alleles for seed color
1/4 TG 1/4 tg 1/4 Tg 1/4 tG
Chromosomes can line up in two ways
during meiosis I
T
Tg gt
GG
T T
T T
T T
t t
t t
t t
t
G G
G G
G G
g g
g g
g gg g
g g
g g
T T
T T
T T
t t
t t
t t
G G
G G
GG
Mendel’s Principle of Independent Assortment
Mendel’s Principle of Independent Assortment
Genes located on one chromosome are sorted for distribution into gametes independently of genes located on another chromosome
Each pair of alleles assorts independently of the other pairs
All possible combinations of alleles can occur in the gametes
Calculating Allele Combinations
What are the possible allele combinations that can occur in the offspring from the following crosses? AaBb x AaBb AABB x AaBb
AaBbDd x AaBbDd
Using a Punnett square when considering crosses of 3 or more traits is cumbersome!
Calculating Allele Combinations
Using a Punnett square when considering crosses of 3 or more traits is cumbersome!
Consider another way… Determine the fraction of each individual set of
alleles like in a monohybrid cross. The probability of a particular combination of alleles
is equal to the product (multiplication) of the individual alleles.
Apply Your Knowledge
What fraction of the offspring of parents, each with the genotype KkLlMm, will be kkllmm? Fraction of kk? Fraction of ll? Fraction of mm?
Suppose two DdEeFfGgHh individuals are mated. What would be the predicted frequency of ddEEFfggHh offspring from such a mating?
1/4 1/4 1/4 1/64x x =
An organism has a dominant phenotype…Example: purple flowers
What is the genotype of the organism?
Need to perform a testcross!
Testcrosses
Testcrosses
Testcross – method used to determine the genotype of an organism that is phenotypically dominant
cross dominant organism with homozygous recessive organism
Example: P__ x pp
What results do you expect if the dominant organism is homozygous dominant? Heterozygous?
Two-Trait Testcross
L = long wingsl = vestigal (short) wingsG = gray bodyg = black body
What are the possible genotypes of these flies?
Two-Trait Testcross
Perform a testcross:
G__ L__ x ggll
Two-Trait Testcross
L = long wingsl = vestigal (short) wingsG = gray bodyg = black body
If this fly as the genotype GGLL, what are the genotypes of all of the possible gametes
it can produce?
What are the expected phenotypes of offspring
resulting from aGGLL x ggll cross?
Two-Trait Testcross
L = long wingsl = vestigal (short) wingsG = gray bodyg = black body
If this fly as the genotype GgLl, what are the genotypes of all of the possible gametes it can
produce?
What are the expected phenotypes of offspring
resulting from aGgLl x ggll cross?
Two-Trait Testcross
L = long wingsl = vestigal (short) wingsG = gray bodyg = black body
If this fly as the genotype GgLl, what are the genotypes of all of the possible gametes it can
produce?
What are the expected phenotypes of offspring
resulting from aGgLl x ggll cross?
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