The Genetics of Inheritance

The Genetics of Inheritance genetics is the branch of biology dealing with the principles of variation and inheritance inherit means that certain traits are transmitted from one generation to the next traits are distinguishing characteristics that make an individual unique genes are the part of the chromosome that governs the expression of a particular trait alleles are different available versions of the same gene - e.g., eye colour gene comes in brown and blue

Some Early Ideas Hippocrates (460-377 B.P.) every part of the body was involved in the production of “seeds” the seeds would fuse together to make a new person Aristotle (384-322 B.P.) male and female semen mixed upon conception (female important)   William Harvey (1578-1657) new individuals arose through the process of epigenesis embryo grew in stages and was affected by factors both inside and outside of the mother

Some Early Ideas Anton van Leeuwenhoek (1632-1723) used a microscope to examine gametes found “animalcules” in semen and decided they were preformed embryos female contribution was only the influence of the uterus during embryo development   during the 1800’s, the blending theory became popular sperm and egg mixed together resulted in offspring that were a blend of the parent’s characteristics this theory predicted that a plant with red flowers crossed with a plant with white flowers would produce offspring with pink flowers

Some Early Ideas Francis Galton (1870’s) (cousin of Darwin) disproved the theory of pangenesis (still influenced biology 100 years ago) genes formed in every organ of both the female and male body and moved to the gonads    Charles Darwin (1809-1882) offspring had variations of their parents characteristics couldn’t explain why – suggested current science had not yet discovered the puzzle pieces yet  

The Inheritance of One Trait Gregor Mendel (1822-1884) – Father of Genetics studied the common garden pea (Pisum sativum) and determined the fundamental principles of genetics    Proposed Mendel’s laws of heredity peas exhibit different, measurable characteristics that express themselves as one of two traits (ex. characteristic for height could be tall or short)   

The Inheritance of One Trait Mendel’s First Experiment: The Monohybrid Cross Mendel bred tall plants together until the offspring were always tall – a true breeding stock (purebred) the true bred plants became the parent or P generation    he then crossed a true bred tall plant with a true bred short plant the offspring of this cross was the first filial generation or F1 generation   

The Inheritance of One Trait Mendel’s First Experiment: The Monohybrid Cross all of the F1 plants were tall which indicated that the trait for tall plants must be dominant over short dominant traits are always expressed and recessive traits are latent (present but inactive) he formulated the principle of dominance: when individuals with contrasting traits are crossed, the offspring will express only the dominant trait Mendel showed that heredity is not just a blending of traits   

tall x short all F1 were tall The Inheritance of One Trait Mendel’s First Experiment: The Monohybrid Cross tall x short all F1 were tall    letters are used to represent the different alleles T – tall (dominant) t – short (recessive) parent cross TT x tt all F1 generation Tt (tall and short are the different possible alleles)    if both of the alleles are the same, the individual is said to be homozygous if the alleles are different, heterozygous   

The Inheritance of One Trait Mendel’s First Experiment: The Monohybrid Cross TT – homozygous tall plant Tt – heterozygous tall plant tt – homozygous short plant    this type of cross is called a monohybrid cross because only one trait (height )was studied   

The Inheritance of One Trait Mendel’s Second Experiment: The Monohybrid Cross Part Two Mendel then bred the F1 generation together and found that 3 out of 4 plants in the F2 generation were tall while one was short F2 resembled one parent from the P generation 75% of the time and the other 25% of the time this 3:1 ratio is know as the Mendelian Ratio this information led to the formation of the Law of Segregation which states that inherited traits are determined by pairs of ‘factors’ and these factors segregate (separate) in the gametes, with one in each gamete (pollen and ovules in plants)

the F2 generation was different The Inheritance of One Trait Mendel’s Second Experiment: The Monohybrid Cross Part Two of course, factors were actually alleles, where one is dominant over the other the F1 generation had a chance of getting only a T from the tall plant and a t from the short plant resulting in plants that were all heterozygous (Tt) the F2 generation was different    3:1 Mendelian Ratio    tall x tall tall tall tall short   

Punnett Squares T T T t t Tt Tt T TT Tt t Tt Tt t Tt tt Punnett squares are used to organize the possible trait combinations and to calculate the probability of inheriting a particular trait it can also tell you the genotype (genetic makeup of an individual) and the phenotype (the appearance of a trait in an organism) P generation cross F1 generation cross father mother father mother T T F2 genotypes: TT, Tt, tt phenotypes: tall, short genotypic ratio 1:2:1 phenotypic ratio 3:1 T t t Tt Tt T TT Tt t Tt Tt t Tt tt

Class Activity Dominant and Recessive Traits

Pedigrees Experimental crosses are not done on humans Scientists study family trees or pedigrees to trace genetic diseases passed on from generation to generation Pedigrees reflect phenotypes

Human Pedigree This shows a brother and sister affected by a disease, along with their maternal grandmother