Things That Affect Heredity

Lesson 14 Things That Affect Heredity I. Heritability a. The improvement of animals through genetics can be either natural or planned 1. Natural: survival of the fittest a. 2. b. They adapt to the environment over time Artificial: animals with desirable traits are used in breeding programs a. Over time, animals show more and more of desired traits. Heritability: the capacity of a trait to be passed down from a parent to offspring. II. Most traits are a combination of selection and environment a. Example: Loin-eye size in pigs is 50% heritable   b. Sow = 5-inch loin eye, Boar = 6-inch loin eye Because of 50% heritability, offspring can only have 5.25 inch loin eye instead of 5.5 inch loin eye Let’s look at a chart that describes some heritability rates. Heritability Rates Estimated Percent Heritability (Taken from Agriscience, Fundamentals & Applications) Trait Cattle Sheep Swine Fertility 0-10 0-15 0-15 Weight of Young at Weaning 15-30 15-20 15-20 Postweaning rate of gain 50-55 50-60 25-30 Fat thickness over loin 40-50 ----- 40-50 Loin-eye area 50-70 ----- 45-50 Percent lean cuts 40-50 ----- 30-40 III. Sex Determination a. b. c. d. Sex of offspring is determined at moment of fertilization. 1. 2. Female mammal has 2 sex chromosomes (XX) Male mammal has 1 sex chromosome (XY) After meiosis, all egg cells have X chromosome, only 1/2 of sperm cells have X chromosome So, sex of animal is determined by male parent. Easily shown in Punnett square: Sex Determination Female Gametes Male Gametes X Y X XX XY X XX XY IV. Inheritance  Simple Mendelian Inheritance: controlled by dominant and recessive paired alleles  Complex Inheritance     Incomplete Dominance Codominance Multiple Alleles Polygenic Traits Incomplete Dominance  Individual displays a trait that is intermediate between two parents.  Occurs when a snapdragon with red flowers is crossed with a snapdragon with white flowers to produce a snapdragon with pink flowers. Neither the red nor the white allele is completely dominant; flowers appear pink because they have less red pigment.  In Caucasians, the child of a straight haired parent and a curly haired parent will have wavy hair. Straight and curly hair are homozygous dominant traits. Wavy hair is heterozygous and is intermediate between straight and curly hair.  Cross a Hampshire pig with a Yorkshire and the result is a Blue Butt. Codominance  Two dominant alleles are expressed at the same time; both forms of the trait are displayed.  One example is the roan coat in horses. A cross between a homozygous red horse and a homozygous white horse results in heterozygous offspring with both red and white hairs in approximately equal numbers, producing the mixed color called roan. Multiple Alleles  Genes with three or more alleles  ABO blood groups, or blood types, are determined by three alleles, IA, IB, and i. The letters A and B refer to two carbohydrates on the surface of red blood cells. The i allele means that neither carbohydrate is present. The IA and IB alleles are both dominant over i, which is recessive. But neither IA or IB is dominant over the other. When IA and IB are both present in the genotype, they are codominant. When traits are controlled by genes with multiple alleles, an individual can have only two of the possible alleles for that gene. Multiple Alleles As this chart shows, combinations of the three different alleles can produce four different blood types, A, B, AB, and O. Notice that a person who inherits two i alleles has type O blood. Human Blood Types Genotypes IAIA or IAi IBIB or IBi IAIB ii Surface Proteins A B A and B None Phenotypes A B AB O Polygenic Trait  Several genes influence a trait; genes for a polygenic trait may be scattered along the same chromosome or located on different chromosomes.  Determining the effect of any one of these genes is difficult. Due to independent assortment and cross-over during meiosis, many different combinations appear in offspring.  Familiar examples include growth rate, fertility, and carcass merit. All of these characteristics have a degree of intermediate conditions between one extreme and the other. VI. Sex-linked Traits       Traits dependent on genes that follow the inheritance pattern of a sex chromosome. Allele is located on the X chromosome. Most sex-linked traits are recessive. Males have only one X chromosome, a male who carries a recessive allele on the X chromosome will exhibit the sexlinked condition. Female will express the recessive condition only if she inherits two recessive alleles. Thus, her chances of inheriting and exhibiting a sex-linked condition are significantly less. Occurrence of color blindness is much more prevalent in males then females. Color blindness is caused by a recessive trait on the X chromosome. About eight percent of the male population has a form of colorblindness, whereas the female population is less then one percent. VII. Heterosis  Performance of offspring that is greater than the average of the parents. Also referred to as hybrid vigor.  Heterozygotes possess greater vigor or are more desirable in other ways, such as producing more milk or growing faster, then either of the two homozygotes that produced the heterzygote.  Often occurs when two purebred animals are bred together.  This is an important concept for producers who are trying to increase production of milk, meat, wool, etc.  Example: Breeding of Black Angus cows to Hereford bull. VIII: Environmental Influence - External  External environment includes temperature, light, altitude, humidity, disease and feed supply.  Brahman cattle can withstand high temperatures and humidity better than others  Scottish Highland cattle can withstand the rigors of extreme cold better than others  Most important external environmental factor is feed supply. VIII: Environmental Influence - Internal  Internal environment includes genetic interactions  Hormones cause them to be different for males and females.  Horn size in mountain sheep  Feather color in peacocks  Also effected by age of organism. VIII: Environmental Influence  Allelic and environmental interactions all influence the degree to which genetic improvement can be made through selection.  If external environment has large effect on production traits, genetic improvement is quite low.  Best fed animals obviously grow faster due to the nutritional status of the animals  An animal must have a suitable environment to reach its genetic potential.