Genetics and Heredity LS Chapter 5

Genetics and Heredity Heredity Genetics The passing of traits from parents to offspring Genetics The study of how traits pass through generations

Gregor Mendel The father of genetics 1800’s (so much biology in the 1800’s!) Austrian (NOT AUSTRALIAN) monk Worked in the monastery garden Background in math and science

Mendel’s Experiment Used pea plants Controlled the way that the pollinated (reproduced) Mated plants for a certain trait until they were true-breeding (a.k.a. purebred) True-breeding organisms always pass their trait to offspring He then mated 2 of the true-breeding plants together to form hybrids Hybrids are offspring of 2 organisms with different traits Example: Purple and white plant mated together, the offspring are hybrids

First Generation Mendel mated 2 true-breeding plants together Each was true-breeding for 1 form of a certain trait Example: True-breeding purple x true-breeding white All of the offspring are considered to be hybrids All of them were purple Mendel hypothesized that the purple was overpowering the white

Mendel’s Second Generation Mendel then took 2 of the offspring (hybrid purple plants) and mated them together For every 3 purple plants, there was 1 white one (3:1 Ratio) The white trait had come back The same thing happened whenever he mated 2 of the hybrids together Not only that, it happened whatever trait he tried (height, seed shape, etc.)

Mendel’s Conclusions From the experiment, Mendel concluded that one trait was masking (covering up) the other He called the trait that covered the other up the dominant trait He called the trait that was covered up by the dominant on the recessive trait Correcting misconception: The dominant trait is not better, nor is it necessarily more common All “dominant” means is that it can cover up the recessive trait

Genes and Alleles While Mendel did not know about DNA and genes, he correctly guessed that something would be found to explain inheritance A gene is a segment of DNA that controls a trait Ex. Gene for flower color The different forms of a trait (ex. purple and white) are controlled by different forms of a single gene, called alleles Example: Gene for flower color has alleles for purple and white One allele is dominant and one is recessive We symbolize the dominant allele with a capital letter and the recessive with a lowercase letter Example: Dominant: A, B, C, D; Recessive: a,b,c,d The gene is a single letter. Dominance and recessiveness are indicated by capital and lowercase.

Practice: Dominant or Recessive Are the following dominant or recessive? A-dominant b-recessive C-dominant D-dominant e-recessive A note on the letters: Exaggerate if the capital and lowercase look similar! Zz

Inheritance of alleles You have 23 pairs of chromosomes in each body cell During meiosis, the chromosomes are sorted into sperm and egg, each with ½ the number of chromosomes as body cells When sperm and egg come together (fertilization), you get 23 chromosomes from mom and 23 from dad This means that, for each trait, you inherit 2 alleles The pairs of chromosomes contain the same genes (segments of DNA) for traits They may differ in alleles though You can get an allele for blue eyes from your mom and brown eyes from your dad

Chromosome from dad Chromosome from mom Allele for blue eyes Allele for brown eyes Gene for eye color

Genotype and Phenotype Genotype=the genes (alleles) that an organism has Example: Aa, AA, aa Phenotype=The expression of the genotype (i.e. the physical appearance of the organism) Example: Blue eyes, tall, wrinkled seeds, etc. Think geno=gene, pheno=physical

Practice: Genotype/Phenotype Indicate whether the follow show a genotype or a phenotype: AA-Genotype Blue-Phenotype Aa-Genotype Bb-Genotype Tall-Phenotype

Homozygous vs. Heterozygous Because we inherit 2 alleles that may be either dominant or recessive, there are 3 combinations that they can take Dominant-Dominant (true-breeding) Ex: AA Dominant-Recessive (a.k.a. hybrid) Ex. Aa Recessive-Recessive (true-breeding) Ex. aa When an organism has 2 of the same alleles for a trait, we call them homozygous “Homo” means “same” When they have 2 different alleles, we call them heterozygous “Hetero” means “different”

Homozygous vs. Heterozygous Cont. We combine the term “homozygous” with whether it is dominant or recessive AA=homozygous dominant aa=homozygous recessive Heterozygous is only one dominant and one recessive Aa=heterozygous aA=heterozygous Note: Usually we’d flip this so the capital is in front because it is dominant The only way for an organism to show the recessive phenotype is to be homozygous recessive If there is even 1 capital letter, they will express the dominant trait

Practice: Homozygous vs. Heterozygous Indicate whether the following are homozygous dominant, homozygous recessive, or heterozygous SS-homozygous dominant Nn-heterozygous uu-homozygous recessive mM-heterozygous, but we should flip them so the capital is in front

Genotype/Phenotype, Homozygous/Heterozygous, Dominant/Recessive Blue eyes (b) are recessive to brown eyes (B). With that in mind answer these questions: If a person has a genotype of Bb, what color eyes do they have? A person has blue eyes (phenotype). What is their genotype? A person is homozygous dominant for eye color. What is their genotype? What is their phenotype?

Predicting Offspring Traits There is no way to know for certain what offspring will look like. However, we can get a probability of what traits they may get. Punnett Square A tool that shows the probability of offspring inheriting a certain trait. Shows all of the ways in which the parents’ genes can combine Note that it shows PROBABILITY, not certainty Thought Experiment: Coin flip If I flip a coin what is the percent chance it will be heads? If it comes up heads, what is the chance that the next flip will also be heads? If it comes up heads 100 times in a row, what is the chance that the 101st flip will also be heads?

Using probability in genetics If a couple is having a child, what is the chance that it will be a boy? If a couple has 3 boys, what is the chance that their next child will be a girl? If there is a 50% chance of a child inheriting a genetic disorder and the couple has 1 healthy child, what is the probability their next child will have the disorder?

How to make a Punnett Square Because there are 2 alleles for each trait for each parent, we will draw a grid that is 2x2 We will then put one parent’s alleles on the top It does not matter which parent (at least to me…) We will then put the other parent’s alleles on the left side These correspond to the possibilities of the sperm and eggs Then, bring the ones on top down each column and the ones on the left across each row A A A A a a A a a a

Example A person with genotype Gg and a person with genotype GG mate. What are the possible genotypes of the children? GG Gg

Another Example Brown hair is dominant to blonde hair. If a man heterozygous for brown hair (Bb) has a child with a blonde woman (bb), what is the chance that the child will have blonde hair? 50%

One more Example Brown eyes are dominant to blue eyes. A man and woman, each heterozygous for brown eyes, are expecting a child. What is the probability that the child will have brown eyes? What about blue eyes? 75% brown 25% blue

You Try It Free earlobes are dominant to attached earlobes. Harry, who is homozygous dominant for earlobes, has a child with Ginny who is homozygous recessive for her earlobes. Answer the following questions: What is Harry’s genotype? What is Harry’s phenotype? What is Ginny’s genotype? What is Ginny’s phenotype? What is/are the possible genotype(s) and phenotype(s) that their children could have? Make sure to prove it with a Punnett Square.

When Mendel’s Laws Don’t Apply There are situations where genes are not simply dominant and recessive Blended (Incomplete Dominance) 2 dominant alleles (human ABO blood types, patterned offspring) Polygenic inheritance (many genes for a single trait) Sex-linked traits (linked to X- and Y-Chromosome) And more For this class, we’ll look at Incomplete Dominance You’ll do the rest in Biology

Incomplete Dominance No dominant allele The offspring who are heterozygous are a blended phenotype Black chicken and white chicken producing a grey chicken. Red flower and blue flower producing a purple flower To do Punnett Square: No dominant and recessive alleles, Don’t have to use capital and lowercase letters (*some people choose to though) Often, choose the first letter of each possible allele (R for red, B for blue. Purple would then be RB) Some will use one letter, with an apostrophe as the other allele (R for red, R’ for blue. Purple would be RR’) I don’t care how you choose to do them.

Incomplete Dominance Punnett Square Very similar to others Draw a grid that is 2x2 We will then put one parent’s alleles on the top We’ll use RR for a red flower We will then put the other parent’s alleles on the left side We’ll use BB for a blue flower Then, bring the ones on top down each column and the ones on the left across each row Any that are RB are blended, so purple B R R B B B R R B B

Practice A species of flower comes in varieties that are red, white and pink. If you cross 2 pink flowers, what is the chance of having: Red offspring? 25% White offspring? 25% Pink offspring? 50%