HESI A2 Exam: Heredity
• What is the difference between phenotype and genotype?
• What did Gregor Mendel’s Law of Segregation prove?
• What does homozygous and heterozygous mean?
• Curly hair (C) is dominant over straight hair (c). An individual has the genotype Cc. Will they have the dominant or recessive trait?
Intro
Phenotype refers to a trait's physical appearance based on an organism's genetic code (genotype). Environmental factors also play a role.
The genotype is the part of the genetic makeup of a cell, and therefore of an organism or individual, which determines a specific characteristic (phenotype) of that cell (or organism/individual).
Genes are factors that control traits. Genes are the units of hereditary and are a locus (or region) of DNA. They determine what traits are expressed in individuals.
Alleles are different forms of a gene (recessive and dominant). Alleles are different forms of a gene linked to specific characteristics. When two different alleles are inherited for a trait but only one allele is expressed, that allele is called the dominant allele. The allele that is not expressed is called recessive.
Law of Segregation
Be familiar with Gregor Mendel's famous pea plant experiment and the resulting Law of Segregation. The law of segregation states that there are two alleles and that half of the total number of alleles are contributed by each parent.
Law of Segregation: The F1 generation is 100% Pp, which means the first generation is dark purple. The F2 generation is 25% PP (homozygous dominant, purple flowers), 50% Pp (heterozygous, purple flowers), and 25% pp (homozygous recessive, white flowers). This means that the white characteristic reappears in the second generation. The phenotype is 3:1, purple to white.
Law of Independent Assortment & Co-dominance
The law of independent assortment states that traits are passed on randomly and are not influenced by other traits.
Co-dominance refers to the expression of both alleles.
Dominant and Recessive Traits
A recessive trait is expressed only if the offspring has two copies of the recessive allele. A dominant trait will be expressed even if only one recessive allele is present. Each parent must have a recessive allele for the recessive trait to appear in offspring. If a child has blue eyes (recessive trait), we know that the father has at least one recessive allele. If the mother is homozygous (two dominant alleles, represented by BB), then there will never be any offspring with blue eyes. If she is heterozygous (Bb), and the father is as well, then there is a chance for a blue eyed child.
A brown eyed mother and brown eyed father both have heterozygous genotypes for brown/blue eyes. Brown eyes (B) are dominant over blue (b). What is the probability that they produce brown eyed offspring?
The father and mother are both heterozygous, so their genotypes are Bb. The dominant trait will be expressed even if only one copy present. This means that the genotype combination of BB and Bb both result in the dominant trait (brown eyes). If you make a Punnett Square, you find that the genotype combinations are BB, Bb, Bb, and bb. The chance that the offspring will have brown eyes is therefore 75 percent and the chance of blue eyed offspring is 25 percent. If this is confusing, please check out the video on Punnett Squares.
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