G7 Ch 5.1-2 - Inheritance

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

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

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)

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