HB 4 C 2 a Mendelian Genetics Genetics

HB 4 C. 2 a Mendelian Genetics

Genetics • Genetics-study of patterns of inheritance and variations in organisms. – Genes control traits of living things by controlling the formation of their proteins • Diploid cells contain two genes for each trait, 1 on maternal chromosome and 1 from paternal chromosome • Two genes can be the same form or different – Forms produce characteristics of each trait. Ex. A gene for plant height might come in the tall form and a short form – Different forms are called alleles. – Two alleles are segregated during gamete formation in Meiosis II

Law of Dominance • The law of dominance states that some alleles are dominant and others are recessive. – Dominant allele will always have that trait expressed (seen) in the organism – Recessive allele will only have the trait expressed when the dominant allele is not present • Since one of the chromosomes comes from each parent they can be homozygous or heterozygous for each trait – Homozygous means two identical alleles for that trait (both dominant or both recessive) – Heterozygous means two different alleles for that trait (one dominant and one recessive)

Genotypes and Phenotypes • Genotype means the genetic makeup of an organism that shows the types of inherited alleles. – Usually represented by capital (dominant) and lowercase (recessive) letters. – TT homozygous dominant genotype – tt homozygous recessive genotype – Tt heterozygous genotype • Phenotype is a description of the way a trait is expressed or the physical characteristics of an organism – organisms with genotypes Tt or TT would have a phenotype of tall – Organisms with genotype tt would have a phenotype of short

Law of Segregation • Law of segregation explains how alleles are separated during meiosis. – Each gamete gets one of the two alleles the parent carries for each trait. So they all have the same chance of getting either one of the alleles for each trait. – When the egg is fertilized, each parent donates one copy of each gene to the offspring.

Diagram of law of segregation

The Law of Independent Assortment • Law of Independent Assortment states that segregation of alleles of one trait does not affect the segregation of alleles of another trait. – Genes on separate chromosomes separate on their own during meiosis. – This is true for all genes unless they are linked. When this happens, genes do not separate on their own when forming a gamete. Usually they are close together on the same chromosome.

Punnett Squares: Predicting Genetics • We can use the principles in genetics to predict inherited traits of offspring. • A Punnett Squares are used to predict possible genetic combinations in offspring from different parental allele combinations. • The simplest of these is a monohybrid crosslooks at the inheritance of one trait. It could be between any possibility of genotypes.

Punnett Square Example • This represents the probable outcome of 2 heterozygous parents with the trait T= dominant tall, t= recessive short (Tt x Tt) • The parents are the F 1 generation and the offspring at the F 2 generation

Punnett Square Example • • The square shows the following genotypes are possible: ○ there is a 1: 4 ratio (25%) that an offspring will carry two dominant alleles. ○ there is a 1: 4 ratio (25%) that an offspring will carry two recessive alleles. there is a 2: 4 or 1: 2 ratio (50%) that an offspring will carry one dominant allele and one recessive allele. The square also shows the following phenotypes are possible: o there is a 3: 4 ratio (75%) that an offspring will express the tall trait. o There is a 1: 4 ratio (25%) that an offspring will express the short trait T TT Tt tt

The Dihybrid Cross • Shows the inheritance of 2 different traits • The following Punnett square example represents the probable outcome of two traits of homozygous parents with the traits for shape and color: R = dominant round, r = recessivewrinkled; Y = dominant for yellow, y = recessive green (rryy x RRYY). • The parents are the F 1 generation; the resulting offspring possibilities are the F 2 generation.

Dihybrid Cross example ry ry RY Rr. Yy Rr. Yy All of the offspring for this generation would predictably have the same genotype, heterozygous for both traits (Rr. Yy). All of the offspring for this generation would predictably have the same phenotype, round and yellow (16/16 will be round and yellow).