Unit 7: Genetics

11-1 The Work of Gregor Mendel11-2 Probability and Punnett Squares11-3 Exploring Mendelian Genetics11-4 Meiosis

11-1 The Work of Gregor Mendel Genetics is the

scientific study of how traits are inherited from one generation to next.

Gregor Mendel was a monk who performed extensive experiments breeding pea plants.

11-1 The Work of Gregor Mendel Mendel knew that to produce new

pea plants fertilization need to occur (when male and female reproductive cells join)

Pea plants have the ability to self-pollinate (the sperm cells in the pollen fertilize the egg cells in the same flower)

Mendel created true-breeding peas - plants that self-pollinated and produced offspring identical to themselves

Diagram of a flower

11-1 The Work of Gregor Mendel To determine how different traits are

inherited Mendel cross-bred plants by preventing self fertilization and pollinating the seed of one plant with the pollen of another

11-1 The Work of Gregor MendelGenes and Dominance

Mendel studied seven different traits in pea plants

Traits: a specific characteristic, such as seed color or plant height

11-1 The Work of Gregor MendelMendel’s First Experiment

Mendel mated plants with two contrasting characteristics together. These plants are called the Parental or “P” generation Round pea plant x a wrinkled pea plant

The result of this cross was called the “F1” or filial 1 generation. He also called these offspring hybrids because they were a mix of the two characteristics.

He observed that all of the plants in the F1 generation had the character of only one of the two parents

11-1 The Work of Gregor MendelConclusions of the first experiment

From this first experiment Mendel concluded: Inheritance is determined by “factors”

that determine traits – today known as genes ▪ Different forms of genes are called alleles

Some alleles are dominant and some alleles are recessive ▪ If an organism has a dominant allele for a

trait it will allows visibly show that trait▪ The recessive trait will only be visible if the

organism does NOT have a dominant allele

12-1 The Work of Gregor MendelSegregation the Second Experiment

Mendel mated the “F1” generation or the hybrids through self-pollination (two hybrids mated together)

He found that in the offspring of this mating (which he called the “F2” generation) the recessive alleles reappeared

Mendel’s Experiments

11-1 The Work of Gregor MendelConclusions of the Second Experiment

The dominant allele masked the recessive allele in the F1 generation

Alleles of the F1 generation were split apart when the plants made their sex cells or gametes – this he called segregation

11-2 Probability and Punnett SquaresProbability: the likelihood that a

particular event will occur. The way in which alleles segregate in

organisms is completely random like a coin flip so the principals of probability apply

11-2 Probability and Punnett Squares Terms for Punnett squares:

Genotype – refers to the alleles an organism has, a recessive allele is written as a lower case letter a dominant allele is written as a capital letter▪ Homozygous – means that an organism

contains two identical alleles▪ Heterozygous – means that an organism

contains two different alleles Phenotype – refers to the physical

appearance of the organism

11-2 Probability and Punnett Squares The Punnett square is used to

determine the possible gene combinations that could result from a genetic cross.

11-1 Probability and Punnett Squares A probability is not a certainty, it is a

prediction. Just because the probability of flipping a

heads in a coin toss is 50% does not mean that if a flip a coin twice I will flip heads only one of the two times.

11-3 Exploring Mendelian GeneticsDihybrid Crosses

Mendel wanted to know if genes segregated together or separately

His next experiment involved following two traits at a time▪ Crossing a homozygous yellow/round pea

plant (RRYY) with a homozygous green/wrinkled pea plant (rryy)▪ The F1 generation produced all yellow/round

pea plants (RrYy)

11-3 Exploring Mendelian Genetics

Dihybrid Crosses Continued Mendel then used the F1 plants and self

pollinated: YyRr x YyRr He found that alleles segregated independently –

use the FOIL (first-outer-inner-last) method to figure out all of the possible combinations of alleles from one parent:▪ YR▪ Yr▪ yR▪ yr

11-3 Exploring Mendelian Genetics The principal of independent

assortment states that genes for different traits can segregate independently during the formation of gametes. Independent assortment helps account for the many genetic variations observed in plants, animals and other organisms.

Exceptions to MendelBeyond Dominant and Recessive Alleles

Incomplete Dominance

Definition: When two or more alleles influence the phenotype and the heterozygous phenotype is a blend of both homozygous phenotypes the trait is considered incompletely dominant.

Incomplete Dominance: Example Problem

The trait for fur color in mice is controlled by two alleles. W for white and w for black. The intermediate phenotype is grey. Cross a grey mouse with a white mouse. What is the probability the offspring will be white?

W wW WW Ww

Codominance

When both alleles for a gene are expressed in a heterozygous offspring the trait is considered codominant (there is no blending in codominant traits both traits are expressed.)

Codominance: Example Problem Two tulip plants are mated one has

yellow petals and the other has red. All the offspring of these flowers have both red and yellow petals. What are the genotypes of the parents and the offspring?

= RR

=YY

= RY

Multiple Alleles

Multiple Alleles: A trait that is controlled by three or more alleles that can fit into one gene.

Multiple Alleles: Example Problem

Example Problem:ABO blood type

Phenotype

Genotype(s)

A typeB typeAB typeO type

Phenotype

Genotype(s)

A type IAIA or IAiB type IBIB or IBiAB type IAIBO type ii

Sex Linkage

Sex Linkage refers to genes that exist only on the X chromosome. Males have only one allele for these traits females have two alleles.

Sex-Linkage: Example Problem Example Problem: Ichthyosis forms scales on the skin

because of the build up of dead skin cells, this genetic disorder is X-linked and recessive.

XI XI x XiYXi Y

XI XIXi XIY

Polygenic

A polygenic trait is one controlled by many genes combined together

Polygenic: Example Problem

Skin color three genes are involved gene A, gene B and gene C, if a person has all dominant alleles i.e. AABBCC they have a very dark skin tone if they have half of the dominant alleles AaBbCc they are medium tone and if they have all recessive alleles aabbcc they are very fair. How many different skin tones are possible?

11-4 Meiosis

In order to validate our current understanding of genetic inheritance two things must occur in sexually reproducing organisms: Each organism must inherit a single

copy of every gene from each parent When an organism produces sex cells

(gametes) each gamete can only contain one set of genes

11-4 Meiosis

Sex cells that have one copy of each set of genes are created through a process known as Meiosis

11-4 MeiosisChromosome Number Sexually reproducing organisms

have two copies of each chromosome type this “pair” of like-chromosomes are called homologous chromosomes.Homologous chromosomes

Tetrad

Crossing over

11-4 MeiosisChromosome Number A cell that contains two copies of every

chromosome is called diploid (2N) in humans N=23. Skin cells Heart cells Nerve cells etc.

A cell that contains one copy of each chromosome type is called haploid (1N). Sperm cell Egg cell

11-4 MeiosisPhases of Meiosis Meiosis is a type of cell division that

reduces the number of chromosomes per cell in half creating haploid cells from diploid cells. Meiosis involves two divisions:▪ Meiosis I – homologous chromosomes

separate▪ Meiosis II – sister chromosomes separate

11-4 MeiosisStages of Meiosis: Meiosis I Prophase I:

chromosomes pair with homologous partner forming tetrads (2 chromosomes, 4 chromatids)

Crossing-over helps to shuffle genes by mixing sets of genes in each homologous pair

Metaphase I: homologous pairs line up in

the center of the cell (double file)

Independent assortment further scrambles genetic material

Anaphase I: homologous chromosomes are pulled to opposite ends of the cell

Telophase I: new nuclei have half the number of chromosomes in comparison with parent cell

Crossing over

Tetrad

11-4 MeiosisStages of Meiosis: Meiosis II No DNA replication

occurs between Meiosis I and Meiosis II

Prophase II Metaphase II:

chromosomes line up in a single file line

Anaphase II: chromosomes split at the centromere – sister chromatids move to opposite ends of the cell.

Telophase II: Meiosis results in four haploid cells.

Comparing Mitosis and MeiosisMITOSIS

Produces 2 cells Cells produced are

genetically identical Produces diploid cells Goal: growth and

healing somatic (body) cells

MEIOSIS Produces 4 cells Cells produced are all

genetically different from each other and parent cell

Produces haploid daughter cells

Goal: create gametes for sexually reproducing organisms