- Slides: 13
Mendel’s Conclusions & Dihybrid crosses DO NOW: 2/27 Objectives: 1. Use a Punnett square to predict the outcome of a dihybrid cross. 2. 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. – 50% chance • 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: Pp. Yy x Pp. Yy • Gametes: reproductive haploid cells (sperm and egg) • One individual heterozygous for 2 traits can produce 4 possible combinations of alleles. • • YP Yp y. P yp
Try a Dihybrid Cross! • Predict the outcome of a dihybrid cross between two pea plants heterozygous for both flower color and seed color (Pp. Yy)
Dihybrid Punnett Square PY PY Py p. Y py PP YY PP Yy Pp YY Pp Yy PP yy Pp Yy Pp yy Pp YY Pp Yy pp YY pp Yy Pp yy pp Yy pp yy
Ratios? PP YY PP Yy Pp YY Pp Yy PP yy 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) For example: 50% Of Gametes Yy 50% Of Gametes
Law of Independent Assortment • Factors for different characteristics are distributed independently from one another. For example: Tt Y y 25% 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)