Mendel’s Laws of Heredity and Dihybrid Crosses Genetics Mendel’s Laws of Heredity and Dihybrid Crosses

Vocabulary Genetics is the study of heredity (the inheritance of biological traits). Test Cross: An individual (Parent 1) of unknown genotype is crossed with an individual (Parent 2) with known genotype. By examining the phenotypes of the offspring (F1 generation) the unknown genotype of Parent 1 can be determined.

Test Cross – An Example In fruit flies, red eyes are dominant to white eyes. How would you determine the genotype of a fly with red eyes?

Test Cross – An Example Hint #1: You must examine the traits of the F1 generation.

Test Cross – An Example Hint #2: To get the F1s you must mate the fly. Which genotype is the only genotype you know just by looking at the phenotype?

Test Cross – Summary Always cross the unknown with a homozygous recessive individual. Examine the traits of the offspring. If even just one offspring shows the recessive trait then the unknown must have been heterozygous. However, you must get enough offspring to reliably conclude this. How many is enough????

Meiosis (Review) Reproductive cells produce haploid gametes (egg or sperm) During meiosis, the homologous chromosomes separate into different cells Therefore, each gamete receives only one allele for each type of trait When the gametes (egg and sperm) combine to form an offspring, 2 alleles are inherited (1 from the egg and 1 from the sperm) for each trait. ie: The Law of Segregation

Mendel’s 1st Law The Law of Segregation Organisms inherit two copies of each gene (one from each parent) Organisms donate only one copy of each gene in their gametes. During meiosis the two copies of each gene separate (“segregate”) during gamete formation.

Mendel’s 1st Law – Explained The Law of Segregation means that if your mom was heterozygous for a particular trait, she could have given you the dominant allele or she could have given you the recessive allele. You only get one from your mom. (Same thing regarding alleles from your dad.)

Mendel’s 2nd Law The Law of Independent Assortment Different traits are determined by different genes. Genes for different traits can be inherited separately from other traits.

Mendel’s 2nd Law – Explained The Law of Independent Assortment means that just because you are tall does not mean you have to have brown eyes… Your alleles for a certain trait are not necessarily linked to your alleles for a different trait. They can get shuffled around during meiosis (gamete formation).

Mendel’s 2nd Law – Example In Fruit flies: Red eyes are dominant to white eyes Having wings is dominant to no wings R = red eyes r = white eyes N = wings n = missing wings

Mendel’s 2nd Law – Example R = red r = white N = wings n = missing wings What is the genotype of a heterozygous red eyed fly? What is the genotype of a heterozygous normal winged fly?

Mendel’s 2nd Law – Example R = red r = white N = wings n = missing wings What gametes will a heterozygous red eyed fly produce? What gametes will a heterozygous normal winged fly produce?

Mendel’s 2nd Law – Example R = red r = white N = wings n = missing wings What is the genotype of a fly that is heterozygous for eye color and for wing type?

Mendel’s 2nd Law – Example R = red r = white N = wings n = missing wings What gametes will a fly heterozygous for eye color and wing type produce? (Hint: Use FOIL method.)

Dihybrid Cross A dihybrid cross examines the inheritance of two traits at the same time. We will now examine a dihybrid cross between two flies heterozygous for both eye color and wing type.

Dihybrid Cross – Step #1 1) Assign allele letters to represent the traits R = red r = white N = wings n = missing wings

Dihybrid Cross – Step #2 & 3 2) Determine genotypes of parents. 3) Determine the gametes they will produce. Parents RrNn Gametes RN Rn rN rn

Dihybrid Cross – Step #4 4) Fill in your Dihybrid Punnett Square. Place gametes on outside and F1s on inside just like before. RN Rn rN rn RN Rn rN rn

Dihybrid Cross – Step #4 4) Fill in your Dihybrid Punnett Square. Place gametes on outside and F1s on inside just like before. RN Rn rN rn RRNN RN Rn rN rn

Dihybrid Cross – Step #4 4) Fill in your Dihybrid Punnett Square. Place gametes on outside and F1s on inside just like before. RN Rn rN rn RRNN RRNn RrNN RrNn RN Rn rN rn

Dihybrid Cross – Step #4 4) Fill in your Dihybrid Punnett Square. Place gametes on outside and F1s on inside just like before. RN Rn rN rn RRNN RRNn RrNN RrNn RRnn Rrnn RN Rn rN rn

Dihybrid Cross – Step #4 4) Fill in your Dihybrid Punnett Square. Place gametes on outside and F1s on inside just like before. RN Rn rN rn RRNN RRNn RrNN RrNn RRnn Rrnn rrNN rrNn RN Rn rN rn

Dihybrid Cross – Step #4 4) Fill in your Dihybrid Punnett Square. Place gametes on outside and F1s on inside just like before. RN Rn rN rn RRNN RRNn RrNN RrNn RRnn Rrnn rrNN rrNn rrnn RN Rn rN rn

Dihybrid Cross – Step #5 5)Determine the gametes that give rise to the same phenotypes and write the phenotype ratio. RN Rn rN rn RRNN RRNn RrNN RrNn RRnn Rrnn rrNN rrNn rrnn RN Rn rN rn

Dihybrid Cross – Step #5 5)Determine the gametes that give rise to the same phenotypes and write the phenotype ratio. RN Rn rN rn RRNN RRNn RrNN RrNn RRnn Rrnn rrNN rrNn rrnn Red w/wings = Red w/o wings = White w/wings = White w/o wings = RN Rn rN rn

Dihybrid Cross – Step #5 5)Determine the gametes that give rise to the same phenotypes and write the phenotype ratio. RN Rn rN rn RRNN RRNn RrNN RrNn RRnn Rrnn rrNN rrNn rrnn Red w/wings = Red w/o wings = White w/wings = White w/o wings = RN Rn rN rn

Dihybrid Cross – Step #5 5)Determine the gametes that give rise to the same phenotypes and write the phenotype ratio. RN Rn rN rn RRNN RRNn RrNN RrNn RRnn Rrnn rrNN rrNn rrnn Red w/wings = Red w/o wings = White w/wings = White w/o wings = RN Rn rN rn

Dihybrid Cross – Step #5 5)Determine the gametes that give rise to the same phenotypes and write the phenotype ratio. RN Rn rN rn RRNN RRNn RrNN RrNn RRnn Rrnn rrNN rrNn rrnn Red w/wings = Red w/o wings = White w/wings = White w/o wings = RN Rn rN rn

Dihybrid Cross – Step #5 5)Determine the gametes that give rise to the same phenotypes and write the phenotype ratio. RN Rn rN rn RRNN RRNn RrNN RrNn RRnn Rrnn rrNN rrNn rrnn Red w/wings = Red w/o wings = White w/wings = White w/o wings = RN Rn rN rn

Dihybrid Cross – Step #5 5)Determine the gametes that give rise to the same phenotypes and write the phenotype ratio. RN Rn rN rn RRNN RRNn RrNN RrNn RRnn Rrnn rrNN rrNn rrnn Red w/wings = 9 Red w/o wings = White w/wings = White w/o wings = RN Rn rN rn

Dihybrid Cross – Step #5 5)Determine the gametes that give rise to the same phenotypes and write the phenotype ratio. RN Rn rN rn RRNN RRNn RrNN RrNn RRnn Rrnn rrNN rrNn rrnn Red w/wings = 9 Red w/o wings = 3 White w/wings = White w/o wings = RN Rn rN rn

Dihybrid Cross – Step #5 5)Determine the gametes that give rise to the same phenotypes and write the phenotype ratio. RN Rn rN rn RRNN RRNn RrNN RrNn RRnn Rrnn rrNN rrNn rrnn Red w/wings = 9 Red w/o wings = 3 White w/wings = 3 White w/o wings = RN Rn rN rn

Dihybrid Cross – Step #5 5)Determine the gametes that give rise to the same phenotypes and write the phenotype ratio. RN Rn rN rn RRNN RRNn RrNN RrNn RRnn Rrnn rrNN rrNn rrnn Red w/wings = 9 Red w/o wings = 3 White w/wings = 3 White w/o wings = 1 RN Rn rN rn

Dihybrid Cross – Step #5 5)Determine the gametes that give rise to the same phenotypes and write the phenotype ratio. 9 : 3 : 3 : 1 RN Rn rN rn RRNN RRNn RrNN RrNn RRnn Rrnn rrNN rrNn rrnn Red w/wings = 9 Red w/o wings = 3 White w/wings = 3 White w/o wings = 1 RN Rn rN rn