Patterns of Inheritance

Chapter 12

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Early Ideas of Heredity

Before the 20th century, 2 concepts were the basis for ideas about heredity:

-heredity occurs within species

-traits are transmitted directly from parent to offspring

This led to the belief that inheritance is a matter of blending traits from the parents.

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Early Ideas of Heredity

Botanists in the 18th and 19th centuries produced hybrid plants.

When the hybrids were crossed with each other, some of the offspring resembled the original strains, rather than the hybrid strains.

This evidence contradicted the idea that traits are directly passed from parent to offspring.

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Why Mendel chose Pea?

Gregor Mendel chose to study pea plants because:

1. other research showed that pea hybrids could be produced

2. many pea varieties were available

3. peas are small plants and easy to grow

4. peas can self-fertilize or be cross-fertilized

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Mendel’s experimental designMendel’s experimental method:1. produce true-breeding strains for each trait he was studying2. cross-fertilize true-breeding strains having alternate forms of a trait

-perform reciprocal crosses as well3. allow the hybrid offspring to self-fertilize and count the number of

offspring showing each form of the trait

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Monohybrid Crosses

Monohybrid cross: a cross to study only 2 variations of a single trait

Mendel produced true-breeding pea strains for 7 different traits

-each trait had 2 alternate forms (variations)

-Mendel cross-fertilized the 2 true-breeding strains for each trait

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Monohybrid Crosses

F1 generation (1st filial generation): offspring produced by crossing 2 true-breeding strains

For every trait Mendel studied, all F1 plants resembled only 1 parent

-no plants with characteristics intermediate between the 2 parents were produced

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Monohybrid Crosses

F1 generation: offspring resulting from a cross of true-breeding parents

F2 generation: offspring resulting from the self-fertilization of F1 plants

dominant: the form of each trait expressed in the F1 plants

recessive: the form of the trait not seen in the F1 plants

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Monohybrid Crosses

F2 plants exhibited both forms of the trait in a very specific pattern:¾ plants with the dominant form¼ plant with the recessive form

The dominant to recessive ratio was 3 : 1.

Mendel discovered the ratio is actually:1 true-breeding dominant plant2 not-true-breeding dominant plants1 true-breeding recessive plant

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Monohybrid Crosses

gene: information for a trait passed from parent to offspring

alleles: alternate forms of a gene

homozygous: having 2 of the same allele

heterozygous: having 2 different alleles

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Monohybrid Crosses

genotype: total set of alleles of an individual

PP = homozygous dominant

Pp = heterozygous

pp = homozygous recessive

phenotype: outward appearance of an individual

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Monohybrid Crosses

Principle of Segregation

Two alleles for a gene segregate during gamete formation and are rejoined at random, one from each parent, during fertilization.

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Monohybrid Crosses

Some human traits are controlled by a single gene.-some of these exhibit dominant inheritance-some of these exhibit recessive inheritance

Pedigree analysis is used to track inheritance patterns in families.

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Dihybrid Crosses

Dihybrid cross: examination of 2 separate traits in a single cross

-for example: RR YY x rryy

The F1 generation of a dihybrid cross (RrYy) shows only the dominant phenotypes for each trait.

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Dihybrid crosses

The F2 generation is produced by crossing members of the F1 generation with each other or allowing self-fertilization of the F1.-for example RrYy x RrYy

The F2 generation shows all four possible phenotypes in a set ratio:9 : 3 : 3 : 1

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Dihybrid Crosses

Principle of Independent Assortment

In a dihybrid cross, the alleles of each gene assort independently.

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Probability – Predicting Results

Rule of addition: the probability of 2 mutually exclusive events occurring simultaneously is the sum of their individual probabilities.

When crossing Pp x Pp, the probability of producing Pp offspring is probability of obtaining Pp (1/4), PLUSprobability of obtaining pP (1/4)¼ + ¼ = ½

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Probability – Predicting Results

Rule of multiplication: the probability of 2 independent events occurring simultaneously is the PRODUCT of their individual probabilities.

When crossing Rr Yy x RrYy, the probability of obtaining rr yy offspring is:probability of obtaining rr = ¼probability of obtaining yy = ¼probability of rr yy = ¼ x ¼ = 1/16

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Testcross

Testcross: a cross used to determine the genotype of an individual with dominant phenotype

-cross the individual with unknown genotype (e.g. P_) with a homozygous recessive (pp)

-the phenotypic ratios among offspring are different, depending on the genotype of the unknown parent

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Extensions to Mendel

Mendel’s model of inheritance assumes that:

-each trait is controlled by a single gene

-each gene has only 2 alleles

-there is a clear dominant-recessive relationship between the alleles

Most genes do not meet these criteria.

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Extensions to Mendel

Polygenic inheritance occurs when multiple genes are involved in controlling the phenotype of a trait.

The phenotype is an accumulation of contributions by multiple genes.

These traits show continuous variation and are referred to as quantitative traits.

For example – human height

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Extensions to Mendel

Pleiotropy refers to an allele which has more than one effect on the phenotype.

This can be seen in human diseases such as cystic fibrosis or sickle cell anemia.

In these diseases, multiple symptoms can be traced back to one defective allele.

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Extensions to MendelIncomplete dominance:

the heterozygote is intermediate in phenotype between the 2 homozygotes.

Codominance: the heterozygote shows some aspect of the phenotypes of both homozygotes.

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Extensions to Mendel

The human ABO blood group system demonstrates:

-multiple alleles: there are 3 alleles of the I gene (IA, IB, and i)

-codominance: IA and IB are dominant to i but codominant to each other

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Extensions to Mendel

The expression of some genes can be influenced by the environment.

for example: coat color in Himalayan rabbits and Siamese cats

-an allele produces an enzyme that allows pigment production only at temperatures below 30oC

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Extensions to Mendel

The products of some genes interact with each other and influence the phenotype of the individual.

Epistasis: one gene can interfere with the expression of another gene

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