HiSET Science: Mendel's Laws

Law of Segregation
The law of segregation states that the alleles for a trait separate when gametes are formed, which means that only one of the pair of alleles for a given trait is passed to the gamete. This can be shown in monohybrid crosses, which can be used to show which allele is dominant for a single trait. A monohybrid cross is a genetic cross between two organisms with a different variation for a single trait. The first monohybrid cross typically occurs between two homozygous parents. Each parent is homozygous for a separate allele (gg or GG) for a particular trait. For example, in pea plants, green seeds (G) are dominant over yellow seeds(g). Therefore, in a genetic cross of two pea plants that are homozygous for seed color, the  generation will be 100% heterozygous green seeds.



Monohybrid Cross for a Cross Between Two Gg Parents
If the plants with the heterozygous green seeds are crossed, the  generation should be 50% heterozygous green (Gg), 25% homozygous green (GG), and 25% homozygous yellow (gg).




Law of Independent Assortment
Mendel's law of independent assortment states that alleles of one characteristic or trait separate independently of the alleles of another characteristic. Therefore, the allele a gamete receives for one gene does not influence the allele received for another gene due to the allele pairs separating independently during gamete formation. This means that traits are transmitted independently of each other. This can be shown in dihybrid crosses.

Gene, Genotype, Phenotype, and Allele
A gene is a portion of DNA that identifies how traits are expressed and passed on in an organism. A gene is part of the genetic code.
Collectively, all genes form the genotype of an individual. The genotype includes genes that may not be expressed, such as recessive genes. The phenotype is the physical, visual manifestation of genes. It is determined by the basic genetic information and how genes have been affected by their environment. An allele is a variation of a gene. Also known as a trait, it determines the manifestation of a gene. This manifestation results in a specific physical appearance of some facet of an organism, such as eye color or height. The genetic information for eye color is a gene. The gene variations responsible for blue, green, brown, or black eyes are called alleles. Locus (pl. loci) refers to the location of a gene or alleles.

Dominant and Recessive Genes
Gene traits are represented in pairs with an uppercase letter for the dominant trait (A) and a lowercase letter for the recessive trait (a). Genes occur in pairs (AA, Aa, or aa). There is one gene on each chromosome half supplied by each parent organism. Since half the genetic material is from each parent, the offspring's traits are represented as a combination of these. A dominant trait only requires one gene of a gene pair for it to be expressed in a phenotype, whereas a recessive requires both genes in order to be manifested. For example, if the mother's genotype is Dd and the father's is dd, the possible combinations are Dd and dd. The dominant trait will be manifested if the genotype is DD or Dd. The recessive trait will be manifested if the genotype is dd. Both DD and dd are homozygous pairs. D. is heterozygous.

Dihybrid Cross for the  Generation of a Cross between GGRR and ggrr Parents
A dihybrid cross is a genetic cross for two traits that each have two alleles. For example, in pea plants, green seeds (G) are dominant over yellow seeds (g), and round seeds (R) are dominant over wrinkled seeds (r). In a genetic cross of two pea plants that are homozygous for seed color and seed shape (GGRR or ggRR), the
 generation will be 100% heterozygous green and round seeds (GgRr). If these  plants (GgRr) are crossed, the resulting  generation is shown below. Out of the 16 total genotypes for the cross of green, round seeds, there are only four possible phenotypes, or physical traits of the seed: green and round seed (GGRR, GGRr, GgRR, or GgRr), green and wrinkled seed (GGrr or Ggrr), yellow and round seed (ggRR or ggRr) , or yellow and wrinkled seed (ggrr).

There are nine green and round seed plants, three green and wrinkled seed plants, three yellow and round seed plants, and only one yellow and wrinkled seed plant. This cross has a 9:3:3:1 ratio.




Pedigree
Pedigree analysis is a type of genetic analysis in which an inherited trait is studied and traced through several generations of a family to determine how that trait is inherited.
A pedigree is a chart arranged as a type of family tree using symbols for people and lines to represent the relationships between those people. Squares usually represent males, and circles represent females. Horizontal lines represent a male and female mating, and the vertical lines beneath them represent their children. Usually, family members who possess the trait are fully shaded and those that are carriers only of the trait are half-shaded.
Genotypes and phenotypes are determined for each individual if possible. The pedigree below shows the family tree of a family in which the first male who was red-green color blind mated with the first female who was unaffected. They had five children. The three sons were unaffected, and the two daughters were carriers.