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Mendel’s Conclusions & Dihybrid crosses

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1 Mendel’s Conclusions & Dihybrid crosses
DO NOW: 2/27 Objectives: 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?

2 The Dwarf and the Albino

3 Here’s how you might solve it…
First, find the probability of dwarfism by using a Punnett square. 50% chance Then, find the probability of albinism. Finally, multiply the probability of the two events together to find the probability of both happening 25% chance.

4 Dihybrid crosses The 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 PpYy Gametes: reproductive haploid cells (sperm and egg) One individual heterozygous for 2 traits can produce 4 possible combinations of alleles. YP Yp yP yp

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)

7 Dihybrid Punnett Square
PY Py pY py PY Py pY py PP YY PP Yy Pp YY Pp Yy PP yy Pp yy pp YY pp Yy pp yy

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 chromosome The organisms are diploid

10 Law of Segregation A pair of factors is segregated (separated) during the formation of gametes (sex cells) Y y For example: 50% Of Gametes 50% Of Gametes

11 Law of Independent Assortment
Factors for different characteristics are distributed independently from one another. T T t t Y Y y y For example: 25% Of Gametes 25% Of Gametes 25% Of Gametes 25% 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)

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