Presentation on theme: "Mendel’s Conclusions & Dihybrid crosses"— Presentation transcript:
1 Mendel’s Conclusions & Dihybrid crosses DO NOW: 2/27Objectives:Use a Punnett square to predict the outcome of a dihybrid cross.Define the Mendel’s Law of Segregation and Law of Independent Assortment and explain how they relate to Punnett squares.Task (answer, don’t copy):Dwarfism is caused by a dominant allele (D) in humans, and albinism is caused by a recessive allele (a). If a man heterozygous for dwarfism and albinism reproduces with an albino woman who is not a dwarf, what is the probability they will have an albino dwarf child?
3 Here’s how you might solve it… First, find the probability of dwarfism by using a Punnett square.50% chanceThen, find the probability of albinism.Finally, multiply the probability of the two events together to find the probability of both happening25% chance.
4 Dihybrid crossesThe cross just explained is a dihybrid cross: a cross in which two different traits are studied.The earlier crosses we have studied are called monohybrid crosses because they track only a single trait.
5 Dihybrid Crosses: A Better Way Pea plant cross: PpYy x PpYyGametes: reproductive haploid cells (sperm and egg)One individual heterozygous for 2 traits can produce 4 possible combinations of alleles.YPYpyPyp
6 Try a Dihybrid Cross!Predict the outcome of a dihybrid cross between two pea plants heterozygous for both flower color and seed color (PpYy)
8 Ratios? 9:3:3:1 PP YY PP Yy Pp YY Pp Yy PP yy Pp yy pp YY pp Yy pp yy How many of each?Purple flowers + yellow seeds :Purple flowers + green seeds :White flowers + yellow seeds:White flowers + green seeds:9:3:3:1
9 Mendel’s “Laws” of Heredity The two basic mechanisms of these genetic crosses are explained on the next two slides.It is important to note that these “laws” work under certain conditions:Each gene is on a different chromosomeThe organisms are diploid
10 Law of SegregationA pair of factors is segregated (separated) during the formation of gametes (sex cells)YyFor example:50%Of Gametes50%Of Gametes
11 Law of Independent Assortment Factors for different characteristics are distributed independently from one another.TTttYYyyFor example:25%Of Gametes25%Of Gametes25%Of Gametes25%Of Gametes
12 What do you know?Construct a Punnett square showing a dihybrid cross between two lemmings.One parent is heterozygous for brown fur (Bb: Brown = dominant) and homozygous recessive for a short tail (tt)The second parent is homozygous recessive for white fur and heterozygous for a long tail.Predict how many of the offspring will be double recessive (white fur + short tail)This may be collected!
13 Homework 9.5 AND 3 Dihybrid Practice Problems Big Problem Set due Wednesday, 2.29 (but we will work on it in class tomorrow, 2.28)