GENETICS Lab 8

Objetives Be able to define the following terms a- genes j- locus b- alleles k-chromosome c- homozygous l- sex chrom. d- heterozygous m- carrier e-recessive n- genotype f- dominant o- phenotype g- lack of dominance (incompl) p- probability h- sex-linked genes q- monohybrid i- reciprocal cross r- dihybrid

Be able to interpret genetic data used in a monohybrid cross, and use this data to predict the phenotypes of the next generation. Be able to interpret genetic data used in a dihybrid cross, and use this data to predict the phenotypes of the next generation. Be able to solve genetic problems that involve recessive, dominant, lack of dominance, and linked traits.

The inherited characteristics of a diploid organism are determined at the moment of sperm and egg fusion. The zygote (2n) receives one member of each chromosome pair from each parent. The genetic information that determines the hereditary traits is found in the structure of the DNA molecules in the chromosomes. A short segment of DNA that codes for a particular protein constitutes a gene, a hereditary unit. Both genes and chromosomes occur in homologous pairs in diploid organisms (see diagram below).

In the simplest situation, an inherited trait, such as flower color, is determined by a single pair of genes. The members of a gene pair may be identical (e.g., each codes for red flowers) or may code for a different variation of the trait (e.g., one codes for red flowers, and the other codes for white flowers). Again, in the simplest case, only two forms of a gene exist. Alternate forms of a gene are called alleles

Geneticist use symbols (usually letters like R or r) to represent alleles when solving genetic problems. When both members of a gene pair consist of the same allele, such as RR or rr, the individual is homozygous for the expressed trait. When the members of the gene pair consist of unlike alleles, such as Rr, the individual is heterozygous (hybrid) for the expressed trait.

The genetic composition of the gene pair (e.g., RR, Rr, or rr) is known as the genotype of the individual. The observable (expressed) form of a trait (e.g., red flowers or white flowers) is called the phenotype. An understanding of inheritance patterns enables the prediction of an expected ratio for the occurrence of a trait in the progeny (offspring) of parents of known genotypes.

MENDELS’S PRINCIPLES Gregor Mendel, an Austrian monk, worked out the basic patterns of simple inheritance in 1860, long before chromosomes or genes were associated with inheritance. Mendel’s work correctly identified the existence of the units of inheritance now known as genes.

Mendel proposed two principles concerning the activity of genes, and these principles form the basis for the study of inheritance. In modern terms, these principles may be stated as follows: The Principle of Segregation states that (1) genes occur in pairs and exist un changed in the heterozygous state and (2) members of a gene pair are segregated (separated) from each other during gametogenesis, ending up in separate gametes. The Principle of Independent Assortment states that genes for one trait are assorted (segregated into ghe gametes) independently from genes for other traits

DOMINANT-RECESSIVE TRAITS When a gene pair consists of two alleles, and one is expressed and the other is not, the expressed allele is dominant. The unexpressed allele is recessive. Many traits are inherited in this manner. In the following examples, dominant traits are underlined.

CONDOMINANCE The alleles of some genes are always expressed in the phenotype and are never recessive. Such alleles exhibit condominance. The inheritance of color in snapdragons is a suitable example. When a homozygous red- flowering snapdragon is crossed with a homozygous white-flowering snapdragon, the F1 progeny always have pink flowers since both alleles are expressed.

Sickle-cell anemia in humans is inherited in this manner. The structure of the hemoglobin molecule is controlled by a single gene pair consisting of two alleles, HbA for normal hemoglobin and Hbs for sickle-cell hemoglobin. A person who is homozygous for sickle cell is afflicted with the disease and usually dies early in life. The heterozygote has relatively few abnormal hemoglobin molecules, shows no ill effects, and has an increased resistance to malaria.

DIGYBRID CROSS In garden peas, yellow seed color is dominant over green seed color, and round seed shape is dominant over wrinkled seed shape. The genes for seed shape and seed color are located on separate chromosomes. Consider a cross of a plant producing yellow round seeds with a plant producing green-wrinkled seeds. Both plants are homozygous for both traits (seed color and seed shape). What kinds of seeds will be produced by the progeny?

Based on what is known, you can establish the genotypes of the parents and write out the cross: Yellow-Round x Green-Wrinkled YYRR x yyrr Recalling that the members of a gene pair are separated into different gametes and recognizing that one member of each gene pair must be in each gamete, determine the gametes. Since both parents are homozygous for both traits, each can produce only one type of gamete. Now, set up and complete a Punnett square” Next, mate two of the offspring from the cross (above). Determine the gametes, set up and complete a Punnett square.

Sex-linked Traits In humans, sex is determined by a single pair of sex chromosomes. Females possess two X chromosomes (XX), and males possess an and a Y (XY). The Y chromosome is shorter than the X and lacks some of the genes present on the X chromosome. Those genes that are present on the X chromosome but absent on the Y chromosome are the sex-linked genes that control inheritance of sex-linked traits

For the sex-linked genes, a female is diploid and a male is haploid.Therefore, a recessive allele on the X chromosome of a male will be expressed, whereas the recessive allele must be present on both X chromosomes of a female to be expressed. Two common sex-linked traits in humans are red-green color blindness and hemophilia.

Reciprocal crosses are used to determine the mode (sex-linked or autosomal) of inheritance. The trait in question in the first cross is associated with the female and in the second cross with the male. If the results of both crosses are the same, autosomal inheritance is indicate. If the results are different, sex-linked inheritance is indicated.