Patterns of Inheritance

Chapter 12

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Early Ideas of Heredity

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

5. Produce many “babies”(seeds) FAST!

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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 opposite (Tall crossed with short) pure-bred strains

All F1 plants resembled only 1 parent

-no plants with intermediate forms between the 2 parents were produced (Example: no medium heights)

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Monohybrid CrossesF2 generation: offspring resulting from the self-

fertilization of F1 plants

F2 plants exhibited both forms of the trait:¾ plants with the dominant form¼ plant with the recessive form

Mendel discovered the ratio is actually:1 pure-bred dominant plant2 hybrid dominant plants1 pure-bred recessive plant

Ratio = 3 : 1.

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Monohybrid Crosses

dominant: the form of each trait expressed in the F1 plants (Capital Letters )

recessive: the form of the trait not seen in the F1 plants (Small-case Letters)

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Monohybrid Crosses - definitions

gene: information for a trait passed from parent to offspring

alleles: alternate forms of a gene

homozygous: having 2 of the same allele

(TT – tt – RR – rr – BB – bb)

heterozygous: having 2 different alleles

(Tt – Rr – Bb)

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Monohybrid Crosses - definitions

genotype: total set of alleles of an individual

(Genes - letters)

PP = homozygous dominant

Pp = heterozygous

pp = homozygous recessive

phenotype: outward appearance of an individual (Physical)

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Monohybrid Crosses – Mendel’s conclusions

Principle of Segregation:

Two alleles (homologous chromosomes) separate during gamete (Sperm or Egg) formation

Meiosis Proves Mendel was correct!

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Principle of Independent Assortment:

the alleles of each gene divide into gametes independently of each other

Mendel was mostly right about this

(show on board)

Monohybrid Crosses – Mendel’s conclusions

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Probability – Predicting Results

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

Rr Yy x RrYy, probability of obtaining rr yy offspring is:

probability of rr = ¼probability of yy = ¼probability of rr yy = ¼ x ¼ = 1/16

Punnett Squares• Punnett squares can be used to predict the

outcome of a genetic cross

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Pedigree analysis is used to track inheritance patterns in families.

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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 of offspring are different, depending on the genotype of the unknown parent

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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 shows all four possible phenotypes in a set ratio:

9 : 3 : 3 : 1

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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

Many genes do not meet these criteria!

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Extensions to Mendel

(1) Polygenic inheritance- multiple genes control the phenotype of a trait.

These traits show continuous variation

Examples: Human height, Eyecolor and Skin Color

- Lab Retrievers

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Extensions to Mendel

(2) Incomplete dominance: the heterozygote is intermediate in phenotype between the 2 homozygotes.

Red X white

4/4 pink

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

(4) Multiple alleles: more than 2 possible alleles for a gene

Extensions to Mendel

P generation

F1 generation

F1 generation

F2 generation

C.Multiple alleles: gene with more than 2 possible alleles

1. Each individual inherits only 2

2.Examples: Blood types (A B o) +

Rabbit Coats (C ch h c )

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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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