Introduction to Genetics A quick review ….

What is genetics? The scientific study of heredity

Gregor Mendel Born in 1822 in Czechoslovakia Became a monk in 1843 Taught biology & had interest in statistics Also studied at the University of Vienna

Mendel continued While at the monastery, taught & worked in the garden Between 1856 - 1863, he grew & tested over 28,000 pea plants

Mendel’s pea plants Easy to grow Easily identifiable traits Can work with large numbers of samples

Mendel’s experiments Mendel created “purebred” or true-breeding generations Made sure certain pea plants were only able to self pollinate Eliminated unwanted traits He achieved this by cutting away the stamen, or male part, of each flower

Genes & dominance Trait: a characteristic Mendel studied 7 traits After Mendel ensured his true-breeding generation was pure, he then crossed plants showing contrasting traits White X purple flowers Round X wrinkled peas He called the offspring the F1 generation, or 1st Filial

What will happen when you cross pure yellow peas with pure green peas? All of the offspring were yellow Hybrids = the offspring of crosses between parents with purebred contrasting traits

What did Mendel conclude? Inheritance is determined by factors passed on from one generation to the next Mendel knew nothing about chromosomes, genes, or DNA These terms had not yet been discovered/defined

What were Mendel’s “factors”? “Factors” are genes Each gene has a different form called alleles Mendel’s Law of Dominance Stated that some alleles are dominant and some are recessive

Genotype The genetic makeup of an organism Symbolized with letters Capitalized (T) or lower case (t) Homozygous = when an organism possesses two identical alleles for the same trait TT (homozygous dominant) tt (homozygous recessive) Heterozygous = when an organism possesses different alleles for the same trait Tt

Phenotype The physical appearance of the organism What the organism looks like Expression of the trait Short, tall, yellow, smooth, et cetera

? X = Mendel’s second cross Mendel allowed the F1 generation to self-pollinate, thus producing the F2 generation Did the recessive allele completely disappear? What happened when he crossed two hybrid yellow (F1) pea plants? ? X =

Results ¾ of the peas were yellow, ¼ of peas were green Mendel concluded that during the formation of the sex cells, or gametes, the alleles separated, or segregated, to different gametes (pollen or egg) Mendel’s Law of Segregation

Probability The likelihood of a particular even occurring Chance Can be expressed as a fraction or percent Example: coin flip

Probability & statistics No one event has a greater chance of occurring than another You cannot predict the precise outcome of an individual event The more trials performed, the closer the actual results reflect the expected outcome

Punnett square Developed by Reginald Punnett A diagram used to show the probability, or chances, of a certain trait being passed from one generation to the next

Reading Punnett Squares Capital letters (Y) represent dominant alleles Y = yellow pea color Lower case letters (y) represent recessive alleles y = green pea color Gametes are placed above & to the left of the square Offspring are placed in the square Yy  Y and y gametes Y y YY Yy Y y Yy yy

Punnett Square review

Mendel’s Law of Independent Assortment A cross involving two or more traits Example: color of peas & shape of peas Alleles separate independently of each other during the formation of gametes Anaphase I

Monohybrid cross Both parents are heterozygous (hybrid) for one trait

Dihybrid cross Both parents are heterozygous (hybrid) for two traits Example: RrYy x RrYy A specific type of two-trait problem AaBb x AaBb is a dihybrid cross Aabb x aaBb is not a dihybrid cross

Mendel’s death Mendel published his paper on heredity in 1866 The scientific community saw little, if any, importance in his work Mendel died in 1884 with no recognition for his contributions to genetics

Some exceptions of Mendel’s principles Some alleles are neither dominant nor recessive Many traits are controlled by more than one gene (polygenic)

Incomplete dominance Neither allele is dominant When both alleles are present, a “new” phenotype appears that is a blend of each allele Alleles should be represented by capital letters Use superscripts or prime marks to differentiate Cr = red Cw = white or R’ = red R = white

Four-o-clock flowers Red flower genotype = CrCr (or R’R’) White flower genotype = CwCw (or RR) Pink flower genotype (the heterozygous state) = CrCw (or R’R) Be careful: Some worksheets use RR = red, WW = white, RW = pink … technically, this is WRONG RW = haploid = gamete

What happens when a red flower is crossed with a white flower? CrCr X CwCw All offspring are pink CrCw pink CrCw pink Cw CrCw pink CrCw pink Cw

Codominance When the phenotype of both alleles is fully expressed in the heterozygote CrCr = cattle with red hairs CwCw = cattle with white hairs CrCw = cattle with roan coat (mixture of red & white hairs)

Multiple alleles When more than two alleles, for a single trait, are found in nature Human blood types: A, B, O, and AB A and B are codominant to each other A and B are both dominant over O

Human blood types Type A Allele = IA Blood cells have antigen A on membrane surface Antigen = membrane protein

Human blood types Type B Allele = IB Blood cells have antigen B on membrane surface

Human blood types Type AB Genotype = IAIB Blood cells have both types of antigens on cell membrane Known as the universal recipient

Human blood types Type O Allele = i Blood cells have no antigens on cell membrane Known as the universal donor

Human blood types Genotypes & phenotypes IAIA A blood IAi A blood IBIB B blood IBi B blood IAIB AB blood ii O blood

How common are the different blood types? 45 % 40 % 4 % 11 %

Try it! A sample problem A man with type AB blood marries a woman with type B blood (whose father has type O blood). What are the chances they have a child with type A blood? A child with type AB blood?

Polygenic traits Traits controlled by two or more genes Examples: human height, eye color, skin color, hair color, IQ

Rediscovery of Mendel’s work Early 1900’s many scientists “rediscovered” Mendel’s work 1902 - Sutton 1908 – Garrod 1910 - Morgan

Thomas Hunt Morgan 1866 - 1945 Born in Kentucky, professor of Biology at Columbia University Worked with fruit flies (Drosophila) Nobel Prize in Medicine (1933)

Why do geneticists love the fruit fly? Can work with large numbers of flies easily Produce many offspring Short reproductive cycle Only 4 pairs of chromosomes

Gene linkage Morgan found that many genes are linked together Morgan specifically noticed sex-linkage It was also determined that chromosomes, not genes, assort independently during meiosis

Gene maps & crossing-over First developed by Sturtevant in 1911 The farther apart two genes are, the more likely they will cross-over during meiosis

Any questions?