Mendel’s Laws of Heredity Why we look the way we look...

What is heredity? The passing on of characteristics (traits) from parents to offspring Genetics is the study of heredity

Inheritance Theory Prior to Mendel 1. Traits “blended” Trait: characteristics to be passed from parent to offspring “bloodlines”: thought traits passed through the blood 2. Problem with blending: cannot account for unexpected traits

Mendel- Father of Genetics Personal history 1. Austrian monk 2. Teacher of high school natural science- love of evolution, nature, meteorology 3. “for the fun of it”: crossed peas and mice- saw inheritance patterns 4. pea plants- a formal test

Mendel used peas because: They reproduce sexually They have two distinct, male and female, sex cells called gametes Their traits are distinctive and easy to isolate Reproduce quickly

Mendel crossed them Fertilization - the uniting of male and female gametes Cross - combining gametes from parents with different traits

Steps of Mendel's Experiment

What Did Mendel Find? He discovered different laws and rules that explain factors affecting heredity.

 Mendel studied the inheritance of one trait (for example plant's height, color of  flowers or color and shape of seeds). A cross in which only one trait is studied is called a monohybrid cross.

Mendel first cross pollinated tall pea plants ,identified as TT (height of plants in this variety were about six feet tall ), with each other.

Mendel noticed, that only tall plants were produced Mendel noticed, that only tall plants were produced. He came to a conclusion, that the tall variety of a pea plant, must contain some factor for tallness.

He then crossed short with short and the result was all short. X He then crossed short with short and the result was all short. Once again he concluded that pea plant must contain some factor for height

The next step of Mendel's experiment was to cross tall pea plants (TT) with short pea plants (tt). The resulting plants were labeled Tt And only tall plant were produced

Mendel’s next experiment involved allowing the tall plants from the tall short cross to self pollinate. The result was a mixture of tall and short plants

Mendel named each generation : Starting generation – Mendel named each generation : Starting generation – P (parent) generation. The following offspring generation was called F1 - first filial generation F2 - second filial generation, and so on.

P F1 F2

The results of the crosses were always the same. Mendel experimented with many traits The results of the crosses were always the same.

When pure bred plants were crossed the offspring always had the trait of the parents. When two different alleles for a trait were crossed the offspring always showed the dominant trait. When the offspring of the previous were allowed to self pollinate the recessive trait showed up again.

Rule of Unit Factors -Alleles - different forms Each organism has two alleles for each trait one from each parent -Alleles - different forms of the same gene -Genes - located on chromosomes, they control how an organism develops

Rule of Dominance Some genes (alleles) are dominant and others are recessive. The phenotype (trait) of a dominant gene will be seen when it is paired with a recessive gene. TT & Tt both result in a TALL plant, because T is dominant over t. t is recessive. tt will result in a short plant.

Law of Segregation Each gene (allele) separates from the other so that the offspring get only one gene from each parent for a given trait. TT x tt

Law of Independent Assortment The genes for different traits are inherited independent of each other.

Phenotype & Genotype Phenotype - the way an organism looks red hair or brown hair genotype - the gene combination of an organism AA or Aa or aa

Phenotype Phenotype Physical characteristics

Genotype Phenotype Genotype Physical characteristics Genes we inherit from our parents

Heterozygous & Homozygous Heterozygous - if the two alleles for a trait are different (Aa) Homozygous - if the two alleles for a trait are the same (AA or aa)

Questions... How many alleles does an organism have for each trait? What is an allele? How many alleles does a parent pass on to each offspring for each trait

Questions... What do we call the trait that is observed? What case (upper or lower) is it written in? What about the one that disappears? What case is it written in?

Punnett Square Many years later a method of showing the crosses was developed. It is referred to as the Punnett Square and shows the probability of the offspring having a certain trait.

Punnett Squares t Tt The genes from one parent go here. The genes from the other parent go here.

Punnett Squares T t

Punnett Squares T t Tt

Punnett Squares T t Tt

Punnett Squares T t Tt

Punnett Squares T t Tt

Punnett Squares T t Tt T T T

Punnett Squares T t Tt T T T Offspring

Punnett Squares T t Tt F1 generation

Interpreting the Results The genotype for all the offspring is Tt. The genotype ratio is: Tt - 4/4 The phenotype for all the offspring is tall. The phenotype ratio is: tall - 4/4

Punnett Squares T t ??

Punnett Squares T t TT Tt tt F2 generation

T t TT Tt tt Next, give the genotype and ratios of the offspring (F2 generation). T t TT Tt tt

Punnett Squares T t TT Tt tt Genotype ratio: TT – 1/4

Punnett Squares T t TT Tt tt Genotype ratio: TT - 1, Tt – 2/4

Punnett Squares T t TT Tt tt Genotype ratio: TT – 1/4, Tt – 2/4, tt – 1/4

T t TT Tt tt Next, give the phenotype ratios of the offspring (F2 generation). T t TT Tt tt

Punnett Squares T t TT Tt tt Genotype ratio: TT – 1/4, Tt – 2/4, tt – 1/4 Phenotype ratio: Tall – 3/4

Punnett Squares T t TT Tt tt Genotype ratio: TT – 1/4, Tt – 2/4, tt – 1/4 Phenotype ratio: Tall – 3/4, short – 1/4

This is a monohybrid cross. We worked with only one trait This is a monohybrid cross. We worked with only one trait. The height of the plant. T t TT Tt tt

We crossed two pea plants which contained both tall and short information. Tt TT tt

Questions... What is the phenotype? What is the genotype? What is homozygous? What is heterozygous? What is monohybrid crossing?

Patterns of Inheritance Dominant /Recessive Codominance Incomplete Dominance Multiple Alleles Polygenic X-Linked Maternal

Dominant/Recessive Trait is controlled by two alleles – one of the alleles is dominant , the other recessive. Example : the height of pea plants

Codominance recessive; both alleles are expressed in the offspring The alleles are neither dominant nor recessive; both alleles are expressed in the offspring A hybrid will have a mixture of the alleles, not just one or the other. Symbols for codominant alleles are special Example of chicken feather color (FB = black feathers)(FW = white feathers)

Incomplete Dominance The alleles for a trait blend . An example would be a red flower is crossed with a white flower and the resulting plant produces pink flowers.

Multiple Alleles The trait is controlled by genes that have more than two alleles The organism inherits only two of the alleles Example Human Blood Types – A, B, O Alleles IA, IB, and i

Polygenic Inheritance More than one pair of genes determine the phenotype. Many phenotypes are possible Example: Height, skin color

Sex-Liked Inheritance Genes controlling sex of an organism (X,Y) are not identical in length. The X chromosome is longer than the Y and therefore contains more genes. Males receive only one set of those genes – from their mother. If a recessive allele is received there is no dominant to block it.

Maternal Inheritance Mitochondrial DNA is inherited from mothers because the egg cell has the mitochondria in it

Genetic Disorders Can be caused by mutations in the genes Genetic disorder can result in minor or major health problems Examples: Cystic Fibrosis, Huntington’s Disease, Sickle-cell Disease, Hemophilia, and Down Syndrome