Biology

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Genetic

Mendle’s law

Patterns of Inheritance

• Mendel’s Laws

• Variations on Mendel’s Laws

• The Chromosomal Basis of Inheritance

• Sex Chromosomes and Sex-linked Genes

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The Field of Genetics has Ancient Roots

• Hippocrates (father of medicine): particles from every part of the body travel to eggs and sperm to be passed on

• Aristotle (philosopher): ‘potential’ rather than particles to produce body features

• 19th century biologists: blending- mom and dad’s traits blend like blue and yellow paint

Hippocrates Aristotle

Experimental Genetics Began in an Abbey Garden

• Modern genetics began in 1860s

• Gregor Mendel (monk in what was then Austria- now Czech Republic)

• Parents pass on discrete, heritable factors (1866)

• Heritable factors retain their individuality for generations (no blending)

• Studied garden peas

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• In a typical breeding experiment

– Mendel mated two different, true-breeding varieties, a process called hybridization

• The true-breeding parents

– Are called the P generation

• The hybrid offspring of the P generation

– Are called the F1 generation

– F2 generation comes next

Experimental Genetics Began in an Abbey Garden

• Cross: pollinating a flower of one variety with the pollen of another variety

Mendel chose to work with peas:

– Because they are available in many varieties

– Because he could strictly control which plants mated with which

– Because he could easily start his experiments with varieties that were “true-breeding”

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Some genetic vocabulary

Character: a heritable feature, such as flower color

Trait: a variant of a character, such as purple or white flowers

Gene: a discrete unit of hereditary information consisting of a specific DNA (nucleotide) sequence on a chromosome Allele: alternative version of a gene

Mendel’s Law of Segregation Describes the Inheritance of a Single Characteristic

Performed monohybrid crosses (only 1 trait differs between the varieties)

• When Mendel crossed contrasting, true-breeding white and purple flowered pea plants

» All of the offspring were purple!!!

Mendel discovered:

A ratio of about three to one (3:1) purple to white flowers,

in the F2 generation

When Mendel crossed the F1 plants –

Many of the plants had purple flowers, but some

had white flowers

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Mendel’s Law of Segregation Describes the Inheritance of a Single Characteristic

Mendel developed a hypothesis to explain the 3:1 inheritance pattern that he observed among the F2 offspring

Four Parts:

1. First, alternative versions of genes account for variations in inherited characters which are now called alleles

Allele for purple flowers

Locus for flower-color gene

Homologous pair of chromosomes

Allele for white flowers

Mendel’s Model for Inheritance

Four Parts:

1. First, alternative versions of genes account for variations in inherited characters which are now called alleles

2. Second, for each character an organism inherits two alleles, one from each parent. These alleles may be the same or different!

Locus for flower-color gene

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Mendel’s Model for Inheritance

Four Parts:

1. First, alternative versions of genes account for variations in inherited characters which are now called alleles

2. Second, for each character an organism inherits two alleles, one from each parent. These alleles may be the same or different!

Locus for flower-color gene

3. Third, if the two alleles at a locus differ then one, the dominant allele, determines the organism’s appearance.

The other allele, the recessive allele, has no noticeable effect on the organism’s appearance

4. Fourth, A sperm or egg carries only one allele for each inherited trait because allele pairs separate (segregate) from each other during the production of gametes

Does Mendel’s segregation model account for the 3:1 ratio he observed in the F2 generation of his numerous crosses?

We can answer this question using a Punnett square

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The Punnet Square

• A very convenient tool used in genetics.

• Steps:

– 1. Determine the parents gametes.

• This may be the most important step.

– 2. Match the gametes, and form possible offspring.

– 3. Determine the chances of each type of offspring.

A basic Punnet square:

• Notice that the mother and father are both heterozygous.

• If the dominant sperm fertilizes the dominant egg, then the offspring would be TT. – There is a 25% chance of this

occurring.

– 1 out of 4 will be this.

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Scientists use a “test cross” if they don’t know what the genotype of an individual is.

• What is the:

– Phenotype of a purple pea plant?

– Phenotype of a white pea plant?

– Genotype of a white pea plant?

– Genotype of a purple pea plant?

• How can you find out the unknown genotype of an individual?

• You would want to breed a “known” genotype against your “unknown.”

– The offspring will tell you what the “unknown” is.

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Not Just Flower Color!

Vocabulary – Enough Already!

• Punnett square: used to determine genotypic and phenotypic frequencies

• Phenotype: an organism’s expressed traits (outward appearance)

• Genotype: an organism’s genetic makeup (gene combinations)

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An organism that is homozygous for a particular gene:

– Has a pair of identical alleles for that gene

– Exhibits true-breeding

An organism that is heterozygous for a particular gene:

– Has a pair of alleles that are different for that gene

Homologous Chromosomes Bear the Two Alleles for Each Characteristic

Remember:

Alleles (alternative forms) of a gene reside at the same locus on homologous chromosomes

– Homozygous: both alleles match (either dominant or recessive)

– Heterozygous: one allele is dominant, one is recessive

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Albinism

Failure to synthesize the pigment melanin

From: Introduction to Genetic Analysis, 8th Ed. Griffiths et al.

The Punnet Square

• A very convenient tool used in genetics.

• Steps:

– 1. Determine the parents gametes.

• This may be the most important step.

– 2. Match the gametes, and form possible offspring.

– 3. Determine the chances of each type of offspring.

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A basic Punnet square:

• Notice that the mother and father are both heterozygous.

• If the dominant sperm fertilizes the dominant egg, then the offspring would be TT. – There is a 25% chance of this

occurring.

– 1 out of 4 will be this.

Scientists use a “test cross” if they don’t know what the genotype of an individual is.

• What is the:

– Phenotype of a purple pea plant?

– Phenotype of a white pea plant?

– Genotype of a white pea plant?

– Genotype of a purple pea plant?

• How can you find out the unknown genotype of an individual?

• You would want to breed a “known” genotype against your “unknown.”

– The offspring will tell you what the “unknown” is.

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Independent Assortment is Revealed by Tracking Two Characteristics at Once

• Dihybrid cross: cross individuals differing in two characteristics

• Mendel crossed peas with round, yellow seeds (dominant traits) and peas with wrinkled, green seeds (recessive traits)

• Determined that traits were passed independent of each other (got yellow, wrinkled offspring, for example)

• Law of independent assortment: each pair of alleles segregates independently of the other pairs of alleles during gamete formation

Putting Mendel’s Laws on the Chromosomes!

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Putting Mendel’s Laws on the Chromosomes!

Putting Mendel’s Laws on the Chromosomes!

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Putting Mendel’s Laws on the Chromosomes!

Putting Mendel’s Laws on the Chromosomes!

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Putting Mendel’s Laws on the Chromosomes!

Independent Assortment of Two Genes in the Labrador Retriever

PRA = Progressive Retinal Atrophy

• Coat color and ‘normal’ vision controlled by separate genes • Blanks in the figure can represent either dominate or recessive alleles • Yellow labs have coat color controlled by different gene altogether!

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In many cases, the genotype is not immediately obvious

Geneticists Use The Testcross to Determine Unknown Genotypes

• A testcross allows one to determine the genotype of an organism with the dominant phenotype but unknown genotype

• Crosses an individual with the dominant phenotype with an individual that is homozygous recessive for a trait

Offspring phenotypes tell you the unknown

genotype

Mendel’s Laws Reflect the Rules of Probability

• Probability scale: 0  1

• Event certain to take place: 1; event certain NOT to take place: 0

• Probabilities of all outcomes must add to 1

• Rule of multiplication: multiply probability of two independent events happening (flipping a coin twice, for example)

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Mendel’s Laws Reflect the Rules of Probability

• What about genetics?

• Trihybrid cross: what’s the probability of getting homozygous recessive at all 3 locations (AaBbCc x AaBbCc)?

– Probability aa: ¼

– Probability bb: ¼

– Probability cc: ¼

Probability aabbcc: ¼ x ¼ x ¼ = 1/64

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Genetics Traits in Humans Can be Tracked Through Family Pedigrees

• Many traits simple Mendelian traits:

– Freckles

– Widow’s peak

– Free earlobes

• Pedigree: family tree

• Square: male, circle: female

female

male

male with trait

female with trait

Genetics Traits in Humans Can be Tracked Through Family Pedigrees

Pedigree showing the inheritance of deafness in a family from Martha’s Vineyard

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Many Inherited Disorders in Humans are Controlled by a Single Gene

• Recessive disorders:

– Most human genetic disorders

– Most common lethal disorder: cystic fibrosis (most common among Caucasians)

– Rate of disorders increases with inbreeding

• Dominant disorders:

– Most are non-lethal (dwarfism, extra fingers and toes)

– Lethal examples: Huntington’s Diesase

Many Inherited Disorders in Humans are Controlled by a Single Gene

• Recessive disorders:

– Most human genetic disorders

– Most common lethal disorder: cystic fibrosis (most common among Caucasians)

– Rate of disorders increases with inbreeding

• Dominant disorders:

– Most are non-lethal (dwarfism, webbed fingers and toes, extra fingers and toes)

– Lethal examples: Huntington’s Diesase

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Concept Check

A man and a woman who are both carriers of the Martha’s Vineyard deafness allele have had three children who are not deaf. If the couple has a fourth child, what is the probability that the child will be deaf?

1/4

Variations on Mendel’s Laws – i.e. Complications, Complications!

1. Incomplete Dominance

2. Multiple Alleles for a single gene

3. Pleiotropy – some genes have multiple phenotypic characteristics

4. Polygenic Inheritance – additive effects of 2 or more genes

(Non-Mendelian Genetics)

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Incomplete Dominance in Snapdragons and Carnations

Or, what if Mendel had studied snapdragons?!

• Complete dominance: dominant allele always expressed, recessive allele expressed in

homozygous condition

• Incomplete dominance: heterozygote is intermediate to dominant and recessive alleles

• Classic example: snapdragons/carnations

Incomplete dominance results in intermediate phenotypes

+ =

Incomplete Dominance in Snapdragons and Carnations

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Michael Brown Joseph Goldstein

Nobel Prize (1985) Physiology or Medicine

Cholesterol uptake lead to the discovery of Receptor Mediated Endocytosis

Famial Hypercholesterolemia: Genetic Disorder Death homozygotes ~ 10 to teens heterozygotes < 50

Exceptions to Mendel’s Laws – Incomplete Dominance

Familial Hypercholesterolemia

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Variations on Mendel’s Laws – i.e. Complications, Complications!

1. Incomplete Dominance

2. Multiple Alleles for a single gene

3. Pleiotropy – some genes have multiple phenotypic characteristics

4. Polygenic Inheritance – additive effects of 2 or more genes

(Non-Mendelian Genetics)

Many Genes Have More Than Two Alleles in the Population

• Why only two alleles? Many genes have more

• Example: blood types

• A and B dominant, O recessive

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Variations on Mendel’s Laws – i.e. Complications, Complications!

1. Incomplete Dominance

2. Multiple Alleles for a single gene

3. Pleiotropy – some genes have multiple phenotypic characteristics

4. Polygenic Inheritance – additive effects of 2 or more genes

(Non-Mendelian Genetics)

A Single Gene May Affect Many Phenotypic Characteristics

Pleiotropy: gene influences multiple characteristics

• Example: sickle-cell disease

• Causes red blood cells to become misshapen (sickle shaped) in low oxygen environments

• Heterozygote usually doesn’t suffer much, but has resistance to malaria (homozygous recessive is sensitive to malaria)

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Pleiotropy: gene influences multiple characteristics

• Example: sickle-cell disease

Variations on Mendel’s Laws – i.e. Complications, Complications!

1. Incomplete Dominance

2. Multiple Alleles for a single gene

3. Pleiotropy – some genes have multiple phenotypic characteristics

4. Polygenic Inheritance – additive effects of 2 or more genes

(Non-Mendelian Genetics)

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Polygenic Inheritance of Skin Color

Consider: • Hypothetical example (at least three genes for skin color) • Three (at least!) ‘dark’ skin alleles for each gene • A, B, C incompletely dominant to a, b, c

Polygenic inheritance: Two or more genes influence a single phenotype

Genes and the Environment

Temperature

Soil pH

‘Nature vs Nurture’

Acid Neutral/Alkaline

http://www.tradenets.com/cats/sirulean.htm
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Genes and the Environment

‘Nature vs Nurture’

Exercise and Sun Exposure

• Many human phenotypes are influenced by both genes and environment:

– Risk of heart disease

– Risk of cancer

– Susceptibility to alcoholism and schizophrenia

– In addition to genes, sun affects skin color

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