Inheritance

Chapter 5 Lessons1-2

p.153-170

Heredity• Organisms usually look like their parents.

• Parents pass some of their characteristic, or inherited traits, to their offspring

• Include: hair/fur color, height, eye color

• The passing of inherited traits from parent to offspring is know as heredity.

Genetics• Genetics is the study of heredity.

• Gregor Mendel discovered the rules for inheritance.

Mom Dad

Gene for Eye Color

We have at least two genes for every trait; one from mom and one from dad.

Gregor Mendel The Father of Genetics

• Gregor Mendel was an Austrian monk in the 1800s.

• He experimented with pea plants

• Helped to understand how traits were passed from parent to offspring

Early Ideas about Heredity

• Blending Theory– Each parent contributed “factors” that were

blended in the offspring

Blending theory is not true for most traits.

Mendel’s Experiment

• Mendel did controlled breeding experiments with pea plants

Mendel’s Experiments

• Why pea plants?– Reproduce quickly

– Easily observable traits (flower color, pea shape)

– Mendel could control which pairs of plants reproduced

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Pollination

• Sperm cells from the pollen fertilize egg cells in the pistil

• 2 Types

Self-pollination vs. Cross-pollination

• Self-pollination– Pollen from one plant lands on the pistil of a

flower on the same plant– This is still sexual reproduction

Cross-pollination

• Pollen from one plant lands on the pistil of a flower on a different plant

Mendel’s Experiments: Self-pollination

• Purple flower parent = 100% purple flower offspring

Mendel’s Experiments: Self-pollination

• White flower parent = 100% white flower offspring

Self-pollination Conclusion

• Mendel called these plants true-breeding plants

• True-breeding means that when a true-breeding plant self-pollinates, it always produces offspring with traits that match the parent– White flower parent = 100% white flower

offspring– Purple flower parent = 100% purple flower

offspring

Mendel’s Experiment: Cross-pollination

Conclusion from 1st generation cross

• Purple flower x white flower = 100% Purple flowers

Conclusion from 1st generation cross

• Mendel called these plants hybrids, because they contained both the purple and white genes

Conclusion from 1st generation cross

• Why did only the purple trait show?

Conclusion from 1st generation cross

• Let’s have these hybrid plants self-pollinate

Second Generation (Hybrid) Crosses

• For every three purple flower plant, there was 1 white flower plant

Second Generation (Hybrid) Crosses

• The trait had disappeared in the 1st generation, but reappeared in the 2nd

Hmmmm…I wonder why it

skipped a generation.

OMG! There’s a

3:1 pattern!

Mendel’s Conclusions

• Traits are control by two genetic factors (genes) that are inherited (passed down)

from the parents.

• Not all genetic factors are equal

• Purple flowers appeared even if the plant had one gene for purple and one for white.

Mendel’s Conclusions

• Mendel decided that purple gene blocked the white gene.

• He called it a dominant trait

• The other gene (white) would be called the recessive trait, because it was weaker.

Genetics Today

Genes and alleles

• Today, we know the “genetic factors” that Mendel discovered are genes.

• Genes are sections, or part, of a chromosome that has genetic information for one trait.

• One gene can have different forms (white flowers and purple flowers)

• Different forms of a gene are called alleles

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Phenotype

• Pheno- = “to show”

• Geneticists call how a trait appears, or is expressed, the trait’s phenotype.

• Example: – blue eye/ brown eyes

– Purple flowers/ white flowers

– Black hair/ blonde hair

Genotype

• Geno = genes

• The two alleles that control the phenotype of a trait

• Cannot see an organism’s genotype• Example:

– PP, Pp, pp

– BB, Bb, bb

Symbols for Genotypes

• The alleles for a trait are given a letter symbol based on the dominant form of the trait.

• Example:– Purple flowers / White flowers = P, p– Yellow seeds / Green seeds = Y, y

– Tall stem / Dwarf (short) stem = T, t

Symbols for Genotypes

• An uppercase letter represents the dominant trait

• A lowercase letter represents a recessive trait

• Examples:– Purple flower (P); white flowers (p)– Yellow seeds (Y); green seeds (y)– Green pods (G); yellow pods (g)

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Homozygous means the organism has two alleles that are the same.

Heterozygous means that the organism has two different alleles for

that gene.

Predicting Inheritance with Punnett Squares

Punnett squares

• Diagrams that help to predict possible genotypes and phenotypes of offspring.

How to make a punnett square?

Using Ratios

• Ratio is 3:1 yellow seeds to green

• Does this mean if you grow 4 offspring 3 will have yellow seeds and 1 will make

green seeds?

Using Ratios

• The phenotype of one offspring does not effect the phenotype of the next.

• For each offspring there would be a 75% chance of it having the yellow phenotype and a 25% chance of it having the green

phenotype.

Using Ratios

• However, if you had a lot of time, like Mendel, and you grow 30,000 pea plants,

the ratio would be near 3:1

Pedigrees

• Other than a Punnett square, pedigrees are also used to model inheritance.

• Pedigrees show phenotypes of genetically related family members

Complex Patterns of Inheritance

Types of Dominance• For Mendel’s pea plants, the presence of

one dominant allele produces a dominant phenotype

• However, not all allele pairs have a dominant-recessive interaction

Incomplete Dominance

• Traits appear as a combination

• Definition: The offspring’s phenotype is a combination or mix of the parent’s

phenotype.

Codominance• When both alleles can be observed in a

phenotype

Codominance• When both alleles can be observed in a

phenotype

Codominance

• When both alleles can be observed in a phenotype

Multiple Alleles

• Some genes have more than two alleles

• Human blood types have 3 alleles– IA, IB, i

• This results in 4 blood types– A, B, AB, or O

• IA and IB are codominant

• i is recessive to IA and IB

Polygenic Inheritance

• For Mendel, each trait he studied was determined by only one gene

• However, a trait can be affected by one than one gene

• Definition: Multiple genes determine the phenotype of a trait

• Examples:– Skin color, height, weight