Genetics 3 And 4

Stu dy ing He r edi ty (Ch . 8-3 – 8- 4)

 Punnett squares were invented by Reginald Punnett.  It is a diagram that predicts all of the possible combinations of gametes in a cross.  Punnett squares with 4 squares are used for Monohybrid crosses.

Ge notyp es  Mixing YY with yy will always give you 4 Yy.  Mixing Yy with Yy will always give you:   

¼ YY (Homozygous Dominant) 2/4 Yy (Heterozygous) ¼ yy (Homozygous Recessive)

 If a Phenotype is dominant but the genotype is unknown, a test cross is performed with a homozygous recessive individual.  Probabilities can also predict the expected results of crosses.

 Probability is the likelihood that a specific event will occur.  Probability =# of one kind of possible outcome total # of all possible outcomes

 Ex. One allele for green seed color and One allele for yellow seed color = ½ probability

 To find the probability of two independent events, just multiply them together.  Ex. Probability that two pennies will land on heads.

½ X ½ = ¼

 When several genes influence a trait, the trait is called a polygenic trait.  Ex. Horse with red hair mates with horse with white hair; baby horse has red and white hair.

 If intermediate traits between two parents are expressed, that is called incomplete dominance.  Ex. Red flower with White flower = Pink flower

 Another ex.  Curly hair with Straight hair = Wavy hair.  Codominance happens when both traits are displayed.  Ex. Black Chicken mixed with White chicken  Black and White striped chicken.

 Genes with 3 or more alleles are said to have multiple alleles.  Ex. Human Blood determined by 3 alleles:

 IA, IB, and i (recessive)  People can have A, B, AB, or O blood types.