Mendelian Genetics Unit 3 Notes

What is genetics? The passing of traits to the next generation is called inheritance, or heredity

Gregor Mendel Gregor Mendel performed cross-pollination in pea plants Mendel followed various traits in the pea plants he bred He is known as the “father of genetics”

Mendelian Genetics The parent generation is also known as the P generation The offspring of this P cross are called the first filial (F1) generation The second filial (F2) generation is the offspring from the F1 cross

Mendel’s Studies Mendel studied seven different traits: Seed or pea color Flower color Seed pod color Seed shape or texture Seed pod shape Stem length Flower position

Genes in Pairs Allele Dominant Recessive An alternative form of a single gene passed from generation to generation Dominant One alleles masks the other Recessive The allele that is masked

Dominance An organism with two of the same alleles for a particular trait is homozygous An organism with two different alleles for a particular trait is heterozygous

Genotype and Phenotype An organism’s allele pairs are called its genotype The observable characteristic or outward expression of an allele pair is called the phenotype

Mendel’s Law of Segregation Two alleles for each trait separate during meiosis During fertilization, two alleles for that trait unite Heterozygous organisms are called hybrids

Law of Independent Assortment Random distribution of alleles occurs during gamete formation Genes on separate chromosomes sort independently during meiosis Each allele combination is equally likely to occur

Genetic Recombination The new combination of genes produced by crossing over and independent assortment

Twin Studies Helps scientists separate genetic contributions from environmental contributions Traits that appear frequently in identical twins are at least partially controlled by heredity Traits expressed differently in identical twins are strongly influenced by environment

Punnett Squares

Monohybrid Cross A cross that involves hybrids for a single trait is called a monohybrid cross

Punnett Squares Used to predict the possible offspring of a cross between two known genotypes

Practice!!!

Dominance and Multiple Alleles

Incomplete Dominance The heterozygous phenotype is an intermediate phenotype between the two homozygous phenotypes. It is a mix of physical appearance between the dominant alleles Dominant alleles represented by a capital letter in crosses Example: R or W

Other ways of writing Incomplete Dominance

Codominance When an individual is heterozygous for such traits, both alleles for one trait in an individual are expressed equally In other words, 2 types of alleles that are equally dominant Phenotype of the heterozygote has both traits simultaneously Both alleles are dominant, so they are both represented by a capital letter: B, W or CB, CW (superscript)

Multiple Alleles Blood groups in humans (this shows both multiple alleles and codominance) ABO blood groups have three forms of alleles

Practice!

Sex-Linked Traits

Karyotype Studies (not being tested on) Karyotype—micrograph in which the pairs of homologous chromosomes are arranged in decreasing size Images of chromosomes stained during metaphase Chromosomes are arranged in decreasing size to produce a micrograph

Telomeres (don’t need to write down) Telomere caps consist of DNA associated with proteins Serves a protective function for the structure of the chromosome They might also be involved in aging and cancer

Nondisjunction (not being tested on) Cell division during which sister chromatids fail to separate properly If this occurs during Meiosis I or II then the resulting gametes will not have the correct number of chromosomes Down syndrome: nondisjunction occurred on chromosome 21. The result is short stature, heart defects, and mental disability

Chromosome Inactivation Barr bodies: the inactivated X chromosome in females One X chromosome is randomly inactivated in an early embryonic cell Example: Tortoise shell cats The gene for coat color is on the X chromosome. When the cat is heterozygous in coat color (black/orange) it is born with patches of different colored fur on its body.

Sex-Linked Traits Traits are controlled by genes located on the sex chromosomes Humans: 22 homologous pairs of matching chromosomes called autosomes, and one pair of sex chromosomes (XX or XY) There are genes found on the X chromosome that are not found on the Y chromosome Inheritance of traits determined by these genes is therefore linked to the sex of the individual

X-linked Traits Genes located on the X chromosome that control traits Since males have only 1 X chromosome they are affected by recessive X-linked traits Females are less likely to express a recessive trait because she has two X chromosomes. One of the X chromosomes can mask the affect of the trait.

Sex Determination Sex chromosomes determine an individual’s gender Autosomes are all the other chromosomes that determine everything else. (i.e. Body cells) The Y chromosome determines the sex of the organism XX=Female; XY=Male

When completing crosses… When writing genotypes: Put the trait gene as a superscript to the sex chromosome XRXR; XRY Capital letter represents a dominant allele and lowercase letter represents a recessive allele

Examples (to name a few) Red-Green Color Blindness Hemophilia

Ex: Hemophilia Recessive sex-linked disorder characterized by delayed clotting of the blood. Very rare in females because she would need to have both X chromosomes with the recessive trait.

Types of Genotypes XHXH (female with normal blood clotting) XHXh (female with normal blood clotting, but is a carrier of the trait) XhXh (female with hemophilia) XHY (male with normal blood clotting) XhY (male with hemophilia)

Ex: Red-Green Color Blindness This is a recessive X-linked trait. A male only needs one copy of this allele in order to be colorblind A female would need to have two copies of the recessive allele. Thus it is very rare to find a color blind female