# Mendel’s Conclusions & Dihybrid crosses

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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?

The Dwarf and the Albino

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.

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.

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

Try a Dihybrid Cross! Predict the outcome of a dihybrid cross between two pea plants heterozygous for both flower color and seed color (PpYy)

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

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

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

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

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

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!

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)