Heredity NotesHeredity Notes

HistoryHistory• Gregor Mendel studied pea plants to see how the

traits could be passed down from one generation to another.

• He found that traits were inherited; meaning traits were passed down from parent to offspring. This process of studying the passing of hereditary information is called genetics.

• The factors that control traits are genes. Genes are found in the chromosomes of the cell. Chromosomes are found in the nucleus of the cell.

Gregor MendelGregor Mendel• A chromosome consists of 2 chromatids joined together

at the centromere. On each chromosome the same types of genes are lined up, these are the alleles (examples of allele for eye color are brown or blue, and for hair color blonde or brown). Chromosomes are found in pairs.

• One of the chromosomes is from the male parent one is from the female parent (giving 46 total chromosomes in a human). This came from the fertilization of the egg and the sperm cell for animals, and the pollination of the egg and sperm cells for plants.

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Mendel cont……

In the mid-1800s, the rules underlying patterns of inheritance were uncovered in a series of experiments performed by an Austrian monk named Gregor Mendel.

Mendel's Plant Breeding Experiments

•The alleles can either be recessive or dominate.•Recessive alleles are represented by lower case letters (ex: a, b, e, t)•Dominant alleles are represented by upper case letters (ex: A, B, E, T)•Phenotype – observable traits (brown hair, blue eyes)•Genotype – the alleles (letters, Tt, BB, or ee)

Homozygous – you have the same two allele traits (the same two size of letters: BB, bb, TT, tt, aa, AA)Heterozygous – you have two different allele traits (two different size letters: Aa, Bb, Tt).

Mendelian GeneticsMendelian Genetics• DominantDominant traits-traits- traits that are expressed.traits that are expressed.• RecessiveRecessive traits-traits- traits that are covered up.traits that are covered up.• Alleles-Alleles- the different forms of a characteristic.the different forms of a characteristic.• Punnett Squares-Punnett Squares- show how crosses are show how crosses are

made.made.• Probability-Probability- the chances/ percentages that the chances/ percentages that

something will occur.something will occur.• Genotype-Genotype- the types of genes (Alleles) the types of genes (Alleles)

present.present.• Phenotype-Phenotype- what it looks like.what it looks like.• Homozygous-Homozygous- two of the same alleles.two of the same alleles.• Heterozygous-Heterozygous- two different alleles.two different alleles.

Mendel was fortunate he chose the Garden Pea

•Mendel studied pea plants to find his ideas about genetics. He came up with these ideas:•1) Rule of unit factors – Each organism contains 2 alleles for each trait; they may be both dominant, both recessive, or a combination of both.•Rule of Dominance – The dominant allele over powers the recessive allele if they are found together.

– Example brown eye color is dominate (B), blue is recessive (b).

 

•Law of Segregation – The alleles for each trait must separate when gametes are formed, therefore a parent only passes on one allele for each trait randomly.•If someone has the genotype of BB, then they have brown eyes. If someone has the genotype of Bb, they also have brown eyes. If someone has the genotype of bb, then they have blue eyes.•Law of Independent Assortment – genes for each trait are inherited independently from each other; hair color is not linked to eyes color.

How is it possible to maintain such genetic continuity?

Martin SheenCharlie Sheen

Emilio Estevez Kirk Douglas Michael

Kirk

ChromosomesHomologous chromosome: one of a matching pair of chromosomes, one inherited from each parent.

Sister chromatids are identical

What genetic principles account for the transmission of such traits from parents to offspring?

The Blending Hypothesis of Inheritance

In the early 1800’s the blending hypothesis was proposed. Genetic material contributed by the two parents mixes in a manner analogous to the way blue and yellow paints blend to make green.

What would happen if this was the case?

Law of Dominance

In the monohybrid cross (mating of two organisms that differ in only one character), one version disappeared.

What happens when the F1’s are crossed?

The F1 crossed produced the F2 generation and the lost trait appeared with predictable ratios.

This led to the formulation of the current model of inheritance.

Alleles: alternative versions of a gene.

The gene for a particular inherited character resides at a specific locus (position) on homologous chromosome.

For each character, an organism inherits two alleles, one from each parent

How do alleles differ?

Dominant - a term applied to the trait (allele) that is expressed irregardless of the second allele. Recessive - a term applied to a trait that is only expressed when the second allele is the same (e.g. short plants are homozygous for the recessive allele).

Dominant allele

Recessive allele

Recessive allele

Recessive allele

Probability and Punnett Squares

Punnett square: diagram showing the probabilities of the possible outcomes of a genetic cross

Genotype versus phenotype.

How does a genotype ratio differ from the phenotype ratio?

Punnett squares - probability diagram illustrating the possible offspring of a mating.

Ss X Ss

gametes

Testcross

A testcross is designed to reveal whether an organism that displays the dominant phenotype is homozygous or heterozygous.

Variation in Patterns of Inheritance

Intermediate Inheritance (blending): inheritance in which heterozygotes have a phenotype intermediate between the phenotypes of the two homozygotes

How Does it Work?How Does it Work?

Genotype and Phenotype Practice Problems.

• In hair color for rabbits the dominant trait is black (B) and the recessive trait is brown (b).

• List the 3 different combinations of letters (genotype)

Genotype and Phenotype Practice Problems.

• In hair color for rabbits the dominant trait is black (B) and the recessive trait is brown (b).

• List the 3 different combinations of letters (genotype)

• BB, Bb, bb

Complete the worksheet

• Using what we have discussed complete the rest of the practice problems.