Chapter 11 Mendel & Heredity

Section 11.1 Genetics – study of heredity; how traits are passed from parents to offspring Gregor Mendel “Father of Genetics” History of Lived in a monastery in Austria in 1842 In 1851, moved to the University of Vienna where he studied science and mathematics Returned to monastery, taught, & kept a garden (most famous for studies on peas)

Initial Observation Mendel crossed (mated) purple flowered peas plants with white flowered pea plants. All the offspring had purple flowers. Two of these first offspring were crossed, and some of their offspring had purple flowers, but a few had white flowers!

II. Features of Pea Plants Vocabulary Characters – physical features that are inherited Flower color in peas is a character Trait – one of several possible forms of a character The flower colors of purple or white are traits Hybrid – offspring of a cross between parents with different traits If a purple flowered peas crosses with a white flowered pea, its offspring would be a hybrid

Pea Plant Characters – Mendel observed 7 characters Height of plant’s stem (tall or short) Position of flower on stem (mid-stem or end of stem) Pod Color (green or yellow) Pod Appearance (smooth or bumpy) Seed Texture (round or wrinkled) Seed Color (green or yellow) Flower Color (purple or white)

Pollination – Mendel manually took pollen from an anther (male) & transferred it to the stigma (female). Self-Pollination: a flower will pollinate itself or another flower on the same plant Cross Pollination: a flower will pollinate a flower on a different plant ***Mendel uses both forms of pollination in is experiments.

III. Mendel’s Experiment Mendel first used true-breeding plants. True-Breeding – a plant that is pure for a trait; when self-pollinated, its offspring will always have that same trait A true-breeding purple flowered plant will always produce purple flowered plants! Mendel formed 14 strains that were true-breeding. These true-breeding plants were labeled the P1 generation since they were the original parents.

Two P1 generation plants of contrasting traits were cross-pollinated Two P1 generation plants of contrasting traits were cross-pollinated. Their offspring were labeled the F1 generation. An F1 generation plant was allowed to self-pollinate. Its offspring were labeled the F2 generation.

An Example of Mendel’s Results P1 = True-Breeding Purple X True-Breeding White F1 = 100% were Purple Flowers F2 = 75% Purple flowers & 25% White flowers * = 3 Purple : 1 White Ratio

**Mendel crossed all 14 traits and always received a 3:1 ratio in the F2 generation

Calculating Ratios Probability – chance something will happen Probability = # of times an event happens # of times it could of happened Ex: In an F2 generation, Mendel recorded 705 purple flowered plants and 234 white flowered plants. Purple: happened 705 times out of 939; 705/939 = 75.1 % White: happened 234 times out of 939; 234/939 = 24.9%

Section 11.2 Explaining Mendel’s Experiments Alternate Versions of Genes = alleles For each character, there are 2 possible traits Each trait is controlled by a specific gene Purple flowers is controlled by one gene for flower color & white flowers is controlled by a different gene for flower color The purple gene is called the purple allele

Dominant & Recessive Alleles Dominant Allele – the allele whose trait is always expressed Recessive Allele – not expressed when a dominant allele is present. P = purple allele p = white allele 2 Dominant alleles (PP) = purple flowers 1 Dominant 1 Recessive (Pp) = purple flowers 2 Recessive alleles (pp) = white flowers

We receive one allele for each character from each parent. So 2 alleles are needed to determine our traits

Mendel’s Laws Law of Segregation A pair of alleles are segregated (separated) during meiosis. This means one allele on one chromosome will be separated from the other allele on the other homologous chromosome during meiosis to make gametes.

Law of Segregation

Law of Independent Assortment Alleles are separated randomly and independently into gametes

Section 11.3 Vocabulary Genotype – all the alleles/genes inherited; an individual’s genetic information Example –Pp is the genotype for a purple flowered plant Phenotype – the physical expression of one’s genotype Example – the color purple is the phenotype of a plant with a genotype of Pp.

Homozygous – genotype with 2 identical alleles Example: PP = homozygous dominant pp = homozygous recessive Heterozygous – genotype with 2 different alleles Example: Pp = heterozygous

Punnett Squares – model/tool that predicts the outcomes of a genetic cross Monohybrid Cross – cross between individuals involving one trait (just flower color) Example In pea plants, purple is dominant over white flowers. Determine the probabilities of possible outcomes for a cross between a homozygous dominant plant and a homozygous recessive plant.

Types of Dominance Incomplete Dominance – an allele does not completely dominate another; phenotype will be a blend. Ex: In snapdragons, the red allele (CR) does not completely dominate the white allele (CW). A heterozygote (CRCW) will have pink flowers.

Codominance – both alleles for a trait are fully expressed (spots, AB blood type in humans) A heterozygous plant (CRCW) would display a phenotype of spots 

Multiple Alleles – a trait has 3 or more possible alleles (an individual still has only 2 alleles for the trait). Ex: Blood type in humans. There are 3 alleles a person can inherit: the A allele, B allele, and O allele. Type A = IA IA or IA i Type B = IBIB or IBi Type AB = IAIB (codominance) Type O = ii

Polygenic Inheritance - two or more genes effect a single phenotype/trait Ex: Skin color in humans is an example of polygenic inheritance

Dihybrid Crosses Dihybrid Crosses – crosses between individuals where 2 traits are being tracked

% Purple & Tall % White & Tall % Purple & Short % White & Short Example # 1 In pea plants, purple flowers (P) is dominant over white (p) and tall pea plants (T) are dominant over short (t). PPtt x PpTt % Purple & Tall % White & Tall % Purple & Short % White & Short  

Example # 2 In a species of flowers, white flowers (W) is dominant over yellow (w) and round seeds (R) are dominant over wrinkled seeds (w). WwRr x WwRr % White & Round % Yellow & Round % White & Wrinkled % Yellow & Wrinkled