Probability Ch. 6.5

Lesson Objectives At the end of this lesson, students will be able to… Define a monohybrid cross in terms of parent genetics and traits Use a Punnett square to calculate the possible outcomes of a monohybrid cross Use offspring genotypes to predict parent genotypes

Probability Probability is the likelihood that a particular event/thing will happen. In genetics, we are generally looking at the probability of a certain gene being passed on When would probability of inheriting a gene be very important?

Probability If we are thinking about a single gene with two alleles – we can use a coin as a model (each side represents one allele) When you toss a coin, what is the probability of landing on heads? Tails?

Probability If we are thinking about a single gene with two alleles – we can use a coin as a model (each side represents one allele) The probability of getting heads on a coin toss is 50/50 – the probability of getting tails is also 50/50 So – on any coin toss you have a 50% chance of getting each side

Probability In the same way getting heads/tails on a coin toss is 50/50 – an organism has a 50/50 chance of getting the recessive/dominant allele Other ways we can say 50/50 chance is: 50% chance, 1 in 2 chance, expected half of the time…

Probability If we had 2 coins (2 genes, one from each parent), and we expect to get tails on EACH coin ½ the time… how would we find the chances of getting BOTH coins to land on tails?

Probability If we had 2 coins (2 genes, one from each parent), and we expect to get tails on EACH coin ½ the time… how would we find the chances of getting BOTH coins to land on tails? ½ x ½ = ¼ We can expect to get both tails about 25% of the time (or ¼ of the time)

Monohybrid Cross Mono = One, or single Hybrid = made from two things Monohybrid crosses look at a SINGLE trait of offspring after breeding parents For example: only looking at pea SHAPE in a certain breeding study

Monohybrid Cross If you know the genotype of the parents, the best way to predict the possible outcomes of a monohybrid cross is by using a Punnett Square

Punnett Squares Named after RC Punnett – he developed this strategy to illustrate genetic crosses See page 173: Visual Vocab Parent 1 alleles Offspring Alleles Parent 2 alleles

Practice on Board For each of the following use a Punnett square to show: possible genotypes, percentage of phenotypes, ratio of phenotypes Homozygous-Homozygous (FF x FF, FF x ff, ff x ff) Heterozygous-Heterozygous (Ff x Ff) Homozygous-Heterozygous (FF x Ff, ff x Ff)

PENNY LAB Work on your own or in groups of TWO Every person needs one penny from the back of the room – sit in seats once you have a penny Each person flip coin 10 times and record number of heads and tails you get

Testcross Since dominant traits are expressed in both homozygous and heterozygous genotypes, it can be difficult to know the actual genotype Testcrosses are used to determine the GENOTYPE of a dominant PHENOTYPE

Testcross Example If we had 2 pea plants – one was short and one was tall We know the short plant HAS to be homozygous recessive (tt) We can’t be sure about the tall plant (Tt or TT??) We can breed these two plants and use the offspring ratios to determine the tall parent genotype If the tall parent was TT what would the offspring ratio be? If the tall parent was Tt what would the offspring ratio be?

Practice Problems Monohybrid cross problems

Meiosis and Genetic Variation Ch. 6.6

Lesson Objectives At the end of this lesson, students will be able to… Explain how meiosis creates genetic variation in terms of independent assortment and crossing over Explain why genetic variation is important in terms of adaptation and survival

Sexual Reproduction = Uniqueness In humans, we have 23 PAIRS of chromosomes (so 46 total) We can calculate the possible gamete combinations of ONE person by using 2n Where n equals number of chromosome pairs 223 = 8,000,000 possible UNIQUE gametes!! If TWO people get together to exchange gametes our possibilities increase to 223 x 223 = 70 trillion possibilities!! This is without counting any other sources of diversity

Sexual Reproduction = Uniqueness Meiosis creates genetic variation in two major ways: Independent assortment of chromosomes  during metaphase and anaphase of meiosis, chromosomes are randomly separated into the daughter cells (gametes) Crossing over  During prophase 1 when homologous pairs link up, pieces of DNA is switched from one chromosome to the other – this mixes up the DNA even more

Crossing over – Pg 180 Happens when homologous chromosomes pair up during prophase of meiosis 1 Parts of the chromosome are swapped to create a mix of DNA on each chromosome

(Somewhat) Random Selection Another source of genetic variation is the random selection of mates There is no rule that says you need to make offspring with the closest mate – animals can find mates anywhere What about on a small island???

Island Genetics When a population is limited to a very small mate selection, the genetic variation decreases and everything starts to look the same This is called the FOUNDER EFFECT

Island Genetics What would happen if a type of bird came to this island that only ate red beetles?

Random Selection Not only is the selection of mates somewhat random, the selection of GAMETES is extremely random. Random fertilization  women have 300,000+ eggs and men have billions of sperm, the combination of gametes is completely up to chance

Homework: For tomorrow: 7.1 FAQ #2