Genetics in Longhorns Part 1

Exploring Genotypes and Phenotypes of Longhorn Cattle  Phenotype – describes the physical appearance of a specific genetic trait or characteristic  Genotype – the genetic code consisting of a pair of alleles that describes a inheritable characteristic or trait  DNA - a double helix chain of nucleic acid in a cell that carries genetic and hereditary information  Chromosomes – a strand of DNA that carries genes in linear order  Gene- a unit of inheritable information arranged located within chromosomes  Allele – one member of a pair of genes that determines genetic characteristics  Wild-type Allele – the gene or characteristic that most commonly occurs in the natural environment this allele is identified as wild by a + symbol following it’s letter designation.  Heterozygous - a pair of alleles that contain two different alleles one of which is dominant  Homozygous – a pair of identical alleles  Dominant Trait – a trait that will appear in offspring if one allele is present . This trait will appear in both heterozygous and homozygous gene pairs.  Recessive Trait – a trait that will appear only if two copies of the allele are present. This trait only appears in homozygous gene pairs  Incomplete dominance – an allele that is not completely recessive to the dominant allele  Punnett Square – a square model of the allele genotypes used to predict the outcome of a genetic cross.  All cells contain DNA which is the “blueprint” that describes how an animal will look or act. DNA the genetic information which determines heredity. Genetics is the Science of ½’s. The genetic information for a specific trait is contained in a pair of genes called alleles. One half of the pair comes from each parent. Offspring Sometimes beef ranchers cross longhorn bulls with their cows because longhorn calves are much smaller than beef calves. Smaller calves mean that less cows and calves die during the birthing process. However many beef ranchers don’t want horns in their herd.  The alleles in a the gene pairs can be the same homozygous     or different heterozygous. When they are heterozygous one allele will be dominant and the other recessive. The polled trait is described as HH. The horn allele is described as hh. The polled allele, H, produces no horns, H is completely dominant over the recessive allele, h, that produces horns. Any time the dominant H allele is present the animal will not have horns. Offspring receives one allele from each parent. The combination of these alleles makes up the genotype which determines the phenotype of each offspring. Results : 100% of the offspring’s genotype is heterozygous type Hh. 100 % of the offspring’s phenotype is polled, does not have horns.  We know that 100% of the offspring of a homozygous crosses will not have horns. What happens when we cross heterozygous crosses? Will any of the offspring have horns. Fill out the next Punnett Squares to predict the outcome of crossbreeding heterozygous cattle. What percentages of each genotypes resulted in this cross? What percentage of each phenotypes resulted in this cross? Were the phenotype and genotype results the same? Why or Why not?  Homogeneous crosses result in 100% of the offspring having the same genotype and phenotype.  Heterozygous crosses have a variable result. Genotypes: HH - 25% Hh - 50% hh - 25% Phenotypes : Polled - 75% Horns - 25% Genotypes: Hh - 50% hh - 50% Phenotypes : Polled - 50% Horns - 50% A. Both of this calf’s parents had horns B. This calf’s sire and dam both carry the recessive allele for horns. C. The genotype represented by this calf is hh. D. The genotype represented by this calf is either Hh or HH.