Genetics Vocabulary

Allele Different versions of one gene. An allele is usually represented by a capital or lower case letter. Examples: Pea plants have two alleles for height. One allele is for tall plants (T) the other allele is for short plants (t).

Remember, half your chromosomes came from your mom and half from dad Remember, half your chromosomes came from your mom and half from dad. This is true for any organism that reproduces sexually. This is why one can have more than one allele for a gene. Also there can be many alleles in a population for one gene. Blood type for example. A B O Called Multiple Alleles. Having more than two alleles for a genetic trait.

Dominant The allele that is expressed if different alleles occur together. By expressed I mean seen or used or better yet it is the gene that is transcribed into protein. For example: In humans freckles is the dominant allele, so if one has the dominant allele (F) and the recessive allele (f), they will have freckles.

Recessive The allele that is not expressed if the dominant allele is present. In order to be expressed, both alleles need to be recessive. For example in height of peas the recessive allele is for height is a short plant (t). In order for a plant to be short it needs to have both alleles for shortness. (t t)

Homozygous and Heterozygous Homozygous: the two alleles are the same. TT or tt Heterozygous: the two alleles are different. Tt

Let’s Put Some of these Terms to Use A pea plant is homozygous recessive for height. Its two alleles would be _____ A pea plant is heterozygous for height. Its two alleles would be ____. A pea plant is homozygous dominant for height. Its two alleles would be _____.

Genotype and Phenotype Genotype: Actual alleles or genes present in the individual. (Tt) is the genotype. (the actual genes / letters) Phenotype: The genotype above would be a tall plant. Tall is the phenotype. Phenotype is the physical result of the genes. Another way of explaining phenotype, it is the protein that results from the genotype and its (the protein’s) resulting effect on the body to produce a trait.

P generation P stands for parents. The p generation are the first two individuals crossed in a breeding experiment. These parents are true-breeding or pure breeding, thus they are homozygous. Ex: A tall pea plant crossed with itself always produces tall pea plants. A dachshund crossed with a dachshund always has dachshund.

F1 Generation and F2 Generation F1 Generation is the offspring (the first generation) from the p generation. F2 Generation comes from the F1 Generation being self pollinated with itself. See Figs. 11-4 and 11-5 pp. 311-312

Mendel’s Law of Segregation The first law states that the two alleles for a trait segregate when gametes are formed. Fig. 11-5 and 11-7 What does this mean? During meiosis when the chromosome number is reduced in half, only 1 allele goes into each sex cell. Fig. 11-15

Mendel’s Law of Independent Assortment The alleles of different genes separate independently of one another during gamete formation. P. 317

What does this mean? A gene for height separates independently from one for color. During metaphase in meiosis 1 the chromosomes line up randomly. We now know this applies to genes on different chromosomes or genes that are far apart on the same chromosome. Figs. 11-9 and 11-10 on p. 317

Incomplete Dominance Occurs when an individual displays a trait that is intermediate between two allele types. Tall x short = intermediate/medium height

Example of Incomplete Dominance In snapdragons, a pure-bred red flower is crossed with a white flower, and the offspring are pink flowers. That is because neither red or white allele is completely dominant. Due to this, less red pigment is produced and the flowers appear pink.

Codominance Two dominant alleles are expressed at the same time. This is different from incomplete dominance. The reason is in codominance both dominant traits are displayed.

Example of Codominance Roan coat in horses. A cross between homozygous red and homozygous white coats produces a roan coat, a heterozygous offspring that produces both red and white hair. The mixed color is roan.

Another example Blood Type: There is type A and type B if you have both alleles you will produce both type of blood groups.

Autosomal and Sex Chromosomes Autosomal: refers to any chromosome besides the sex chromosomes. In human these are chromosomes 1-22. Sex chromosomes: chromosomes that determine whether an organism is female or male. In humans: XX = females XY = males

Sex-Linked Traits A trait whose allele is located on the X chromosome. Sex-linked traits are recessive.

Males and Sex Linked Since the alleles are found on the X chromosome, males usually show sex linked traits much more frequently than females. Males only have one X chromosome, so if they receive an X chromosome with the trait, they will exhibit the trait.

Females are carriers Females have two X chromosomes, thus to show the sex linked trait they need to be homozygous recessive. Meaning both of their X chromosomes carry the gene. Females are carriers because they can be heterozygous dominant and still have the recessive allele, which they can pass on to their children.

Sex linked traits happen in other organisms also, not just humans. Sex linked traits are not related just to male or female (sex) characteristics. Example hemophilia is a sex-linked trait, but involves clotting factors in blood.