Phenotypes and Genotypes Mendel’s experiments showed that “yellow” pea plants are not all the same. Two organisms can look alike but have different gene combinations. The way an organism looks is its phenotype. The phenotype of a pea plant is yellow, regardless of its genes. The gene combination an organism contains is its genotype. The genotype of a yellow pea plant can be YY or Yy because yellow color is a dominant trait.

Homozygous and Heterozygous An organism’s genotype cannot always be known from an organism’s phenotype. An organism is homozygous for a trait if the two trait alleles are the same. A true- breeding yellow pea would be homozygous for yellow color (YY). An organism is heterozygous for a trait if the two trait alleles are different. A yellow pea that has one allele for yellow and one for green peas (Yy) is heterozygous for the yellow color trait.

The Law of Independent Assortment Mendel’s 2 nd law states that genes for different traits are inherited independently of each other. This is known as the law of independent assortment. When a pea plant with genotype RrYy produces gametes, the alleles R and r will separate from each other (the law of segregation) as well as from the alleles Y and y (the law of independent assortment).

Punnett Squares - Monohybrid Crosses This Punnett square is two boxes tall and two boxes wide because each parent makes two kinds of gametes. One parent’s genotype is placed across the top. The other parent’s genotype is placed down the left side.

Sample Punnett Squares Problem R = red color flowers and r = white flowers in roses. State the genotype and phenotype of each parent. Show the genotypes and phenotypes of each of the offspring. Which parents and offspring are homozygous? Which are heterozygous?

Sex-Linked Genes In humans, there are 22 pairs or 44 chromosomes called autosomes and 1 pair or 2 chromosomes called sex chromosomes. An X-linked trait is a trait whose gene is only on the X chromosome. Morgan worked with fruit flies and found that certain traits such as eye color seemed related to the sex of the fly. Only males were white-eyed. Eye color was sex-linked in the fruit fly.

Sex-Linked Genes and Pedigree Analysis Hemophilia in Humans is sex-linked.