Chapter 13: Patterns of Inheritance

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Gregor Mendel (1822-1884) Between 1856 and 1863 28,000 pea plants Called the “Father of Genetics"

Site of Gregor Mendel’s

experimental garden in the

Czech Republic

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Mendel stated that physical traits are inherited as “particles” “factors”

Particulate Inheritance

DNA • Only part of DNA in a chromosome codes for

proteins.

• Only 1.5% of human DNA is expressed as protein

• Bacteria have one single circular piece of DNA and 90% of it is translated.

• Some bacteria have an additional circular piece of DNA called a PLASMID. It can move from one bacterium to another through transformation.

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PLASMID

Genetic Terminology

Trait - any characteristic that can be passed from parent to offspring

Heredity - passing of traits from parent to offspring

Genetics - study of heredity

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Genetic Terminology Cont.

Alleles - two forms of a gene (dominant & recessive) alleles are represented by capital or lower case letters (RR) (Rr) (rr)

Dominant - stronger of two genes expressed in the hybrid; represented by a capital letter (R)

Recessive - gene that shows up less often in a cross; represented by a lowercase letter (r)

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Genetic Terminology cont. Genotype - gene combination for a

trait (e.g. RR, Rr, rr)

Phenotype - the physical feature resulting from a genotype (e.g. red, white)

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Genotypes Homozygous genotype - gene

combination involving 2 dominant or 2 recessive genes (e.g. RR or rr); also called pure

Heterozygous genotype - gene combination of one dominant & one recessive allele (e.g. Rr); also called hybrid

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Types of Genetic Crosses

Monohybrid cross - cross involving a single trait e.g. flower color

Dihybrid cross - cross involving two traits e.g. flower color & plant height

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Genotype & Phenotype in Flowers

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Genotype of alleles: R = red flower r = yellow flower

All genes occur in pairs, so 2 alleles affect a characteristic

Possible combinations are:

Genotypes RR Rr rr

Phenotypes RED RED YELLOW

Mendel’s Pea Plant Experiments

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Why peas, Pisum sativum?

Can be grown in a small area

Produce lots of offspring

Produce pure plants when allowed to self-pollinate several generations

Can be artificially cross-pollinated

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Reproduction in Flowering Plants • Pollen contains sperm

– Produced by the stamen • Ovary contains eggs

– Found inside the flower

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Pollen carries sperm to the eggs for fertilization

Self-fertilization can occur in the same flower

Cross-fertilization can occur between flowers

Mendel’s Experimental Methods

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• Mendel hand-pollinated flowers using a paintbrush

– He could snip the stamens to prevent self-pollination

– Covered each flower with a cloth bag

• He traced traits through the several generations

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How Mendel Began Mendel produced pure strains by allowing the plants to self-pollinate for several generations

Eight Pea Plant Traits

Seed shape --- Round (R) or Wrinkled (r) Seed Color ---- Yellow (Y) or Green (y)

Pod Shape --- Smooth (S) or wrinkled (s) Pod Color --- Green (G) or Yellow (g) Seed Coat Color ---Gray (G) or White (g) Flower position---Axial (A) or Terminal (a) Plant Height --- Tall (T) or Short (t)

Flower color --- Purple (P) or white (p)

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Probability & Genetics

• Can be applied to predict the outcomes of genetic crosses.

• Calculating the probablity of making a specific gamete is just like calculating the probability in flipping a coin.

– Tossing heads = 50%

– Making a R gamete from Rr = 50%

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Possible Gametes

• Geneticists can use laws of prob. to predict alleles of the offspring of crosses or matings.

• Predicted results – what you expect to occur

• Observed results – what actually occurs

• One trait: Rr = R can go into a gamete and r can go into a gamete.

• Two traits: RrYy = RY can go into a gamete, Ry, rY or ry can go into gametes. (foil method)

• Think Meiosis 21

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Generation “Gap”

• Parental P1 Generation = the parental generation in a breeding experiment.

• F1 generation = the first-generation offspring in a breeding experiment. (1st filial generation) – From breeding individuals from the P1 generation

• F2 generation = the second-generation • offspring in a breeding experiment.

(2nd filial generation) – From breeding individuals from the F1 generation

Following the Generations

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Cross 2 Pure Plants TT x tt

Results in all Hybrids Tt

Cross 2 Hybrids get 3 Tall & 1 Short TT, Tt, tt

The Punnett Square

Used to help solve genetics problems/

predict outcomes of crosses

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Steps to doing a Monohybrid Cross –A one trait cross

1. Determine the genotypes of the parents genotypes XX x XY

2. Segregate the alleles to determine the possible gametes that can form from each parent

3. X, X and X, Y 4. Draw a punnett square - gamete possibilities

from parents should be placed on the top and left hand side of the square

5. Match them up to complete the square – these are the possible offspring from these parents.

6. Write the Genotypic Ratio = ½ XX: ½ XY 7. Write the Phenotypic Ratio = 50% Female :

50%Male

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P1 (Parental first generation)Monohybrid Cross • Trait: Seed Shape

• Alleles: R – Round r – Wrinkled

• Cross: Round seeds x Wrinkled seeds

• RR x rr

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R

R

r r

Rr

Rr Rr

Rr

Genotypic Ratio: 100% Rr

Phenotypic Ratio: 100% Round seeds

P1 Monohybrid Cross Review

Homozygous dominant x Homozygous recessive

Offspring all Heterozygous (hybrids)

Offspring called F1 generation

Genotypic and Phenotypic Ratios

=100% Rr and 100% Round seeds

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F1 Monohybrid Cross • Trait: Seed Shape

• Alleles: R – Round r – Wrinkled

• Cross: Round seeds x Round seeds

• Rr x Rr

R

r

r R

RR

rr Rr

Rr

Genotypic Ratio:

1RR:2 Rr:1rr

Phenotypic Ratio:

3 Round:1wrinkled

F1 Monohybrid Cross Review

Heterozygous x Heterozygous Offspring:

25% Homozygous dominant RR 50% Heterozygous Rr 25% Homozygous Recessive rr Offspring called F2 generation Genotypic ratio is 1RR:2Rr:1rr Phenotypic Ratio is 3 Round:1wrinkled

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What Do the Peas Look Like?

…And Now the Test Cross

• Mendel then crossed a pure & a hybrid from his F2 generation

• This is known as an F2 or test cross

• There are two possible testcrosses: Homozygous dominant x Hybrid Homozygous recessive x Hybrid

F2 Monohybrid Cross (1st) • Trait: Seed Shape

• Alleles: R – Round r – Wrinkled

• Cross: Round seeds x Round seeds

• RR x Rr

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R

R

r R

RR

Rr RR

Rr

Genotypic Ratio: 1RR:1 Rr

Phenotypic Ratio: 100% Round

F2 Monohybrid Cross (2nd) • Trait: Seed Shape

• Alleles: R – Round r – Wrinkled

• Cross: Wrinkled seeds x Round seeds

• rr x Rr

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r

r

r R

Rr

rr Rr

rr

Genotypic Ratio:

2 Rr:2rr

P.Ratio:

2 Round: 2wrinkled

F2 Monohybrid Cross Review

Homozygous x heterozygous(hybrid)

Offspring: 50% Homozygous RR or rr 50% Heterozygous Rr

Phenotypic Ratio is 1 round:1 wrinkled

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Results of Monohybrid Crosses

• Inheritable factors or genes are responsible for all heritable characteristics

• Phenotype is based on Genotype

• Each trait is based on two genes, one from the mother and the other from the father

• True-breeding individuals are homozygous ( both alleles) are the same

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