Introduction to Mendelian Genetics

Introduction to Mendelian Genetics As we have learned, many genes code for proteins. These proteins may be structural (keratin in hair), enzymes, transport proteins, antibodies etc. Some genes code for proteins that produce visible effects in an organism. These visible characteristics are called traits.

Trait: Skin Colour eg. skin colour is determined by a group of genes that control how much melanin (a brown coloured) pigment is produced in the skin http://www.thezimbabwean. co/news/zimbabwe-news/ 66410/living-with-albinism .html

Trait: Eye Colour eg. People with dark eyes (black or brown) have genes for different proteins than people with light-coloured eyes (blue or green)

Trait: Achondroplasia Dwarfism eg. People with this type of dwarfism have a mutation in a receptor protein for a growth factor in bone http://www.tjhsst.edu/~jleaf/disability/ jweb/JWEB.HTM

The traits (visible characteristics) that we see are caused by our genes. In genetics, the visible (physical) traits are called the “phenotype”. The genes that are responsible for producing the visible traits are called the “genotype”.

Genotypes and Phenotypes People usually have two copies of each chromosome, and therefore two copies of each gene, one from each parent. This is their genotype. The genes for each trait interact to produce the phenotype (visible characteristics) that we see. The connections between the genotype and phenotype are not always obvious.

eg. brown eyed parents may have blue-eyed children

eg. brown skinned parents can have white skinned children http://www.dailymail.co.uk/femail/article-1197836/Pictured-The-mixed-race-couple-rainbow-children.html

eg. brown haired parents can produce red haired children http://presurfer.blogspot.com/2007_11_01_archive.html

eg. parents with achondroplasia dwarfism may have children with normal height http://biodwarfism.blogspot.ca/2009_02_01_archive.html

eg. puppies from one litter can have very different coat colours http://www.petsplace.co.za/danielsharpeis.htm

Mendelian Inheritance Up until the late 1800s, it was believed that the characteristics of the parents were blended in their offspring. eg. If one parent was tall and one parent was short, the children would be medium height But in the mid 1800s, an Austrian monk named Gregor Mendel performed some very careful experiments and came up with another idea.

Mendelian Inheritance Mendel studied the common garden pea. He noticed that pea plants had either: Green or yellow seeds Green or yellow pods Tall or short stems White or purple flowers Smooth or wrinkled seeds Smooth or wrinkled pods

In his first experiment, Mendel took purebred (true-breeding) plants for one trait: eg. tall plants that always produced tall plants eg. short plants that always produced short plants Mendel pollinated (crossed) the tall plant flowers with pollen from short plant flowers, and visa versa He called this the Parental generation (P).

All the pea plants were tall (100% tall)!!! It was believed that this cross would produce a blend of traits, so the offspring would all be medium height. But, Mendel always obtained the same results: All the pea plants were tall (100% tall)!!! 16

When he crossed purebred purple-flowered pea plants with purebred white-flowered pea plants, the offspring were: All purple-flowered!! When he crossed purebred yellow-pod pea plants with purebred green-pod pea plants, the offspring were: All green-podded!!

Mendel’s First Experiment There was no blending of traits!!

Mendel called these peas the first filial (F1) generation. He called the F1 plants hybrids. The experiment was called a “monohybrid cross” because only one trait was different between the plants. Mendel concluded that some traits are stronger or “dominant” over other traits, and the dominant traits would always be seen. He called this the Principle of Dominance. 19

Then Mendel did a second experiment: Mendel allowed the hybrid (F1) plants to self-pollinate. He collected the seeds and grew the plants to see what the F2 generation looked like. He saw:

Mendel kept very careful records of the numbers of each type of plant. Again, there was no blending of traits. Even though the F1 plants were all tall, there were some short offspring in the F2 generation The short plants had “skipped” a generation. There were: 787 tall and 277 short (74% or ¾) (26% or ¼)

Mendel repeated this experiment to see the F2 generation of other traits The ratio of approximately ¾ to ¼ was repeated for each of the traits he studied. Mendel interpreted the results to indicate that each plant has two “factors” that determine one trait. One factor is dominant over the other. Mendel’s factors were later identified as ‘genes’

Mendels’ Law of Segregation Mendel suggested that the offspring get one factor (gene) from each parent The factors (genes) for each trait are passed on independently of the other traits because they are segregated on different chromosomes This was later explained by the process of meiosis, when the chromosomes and sister chromatids go randomly to either side of the equator during metaphase I and metaphase II

Terminology Genes can exist in different forms. These different forms of the same gene are called alleles. Every organism inherits one allele from each parent. Homozygous – both alleles are the same (HH) Heterozygous – both alleles are different (Hh)

Dominant and Recessive Genes If a dominant allele (version of the gene) is present, it is always “expressed” or visible as a trait in the organism eg. pea plants with T T (tall – tall) genotype are tall (have the tall trait or phenotype) eg. pea plants with T t (tall – short) genotype are tall (have the tall trait or phenotype)

Dominant and Recessive Genes If a recessive allele (version of the gene) is present, it is only “expressed” or visible if there is no dominant allele to over-shadow it: eg. pea plants with T t (tall – short) genotype are tall (have the tall trait or phenotype) eg. pea plants with t t (short – short) genotype are short (have the short trait or phenotype)

The genotype of an individual is the genetic makeup (which alleles it has). The phenotype of an individual is the physical appearance (its traits). eg. TT – the genotype is homozygous dominant – the phenotype (trait) will be tall eg. Tt – the genotype is heterozygous eg. t t – the genotype is homozygous recessive – the phenotype (trait) will be short