Genetics Chapter 11: sec 11-1, 11-2, & 11-3

Genetics (sec 11-1 & 11-2) Genetics –The study of heredity Genes –The chemical factors of inheritance located on chromosomes, made of DNA

Mendelian Genetics How Are Traits Inherited?

Gregor Mendel Raised on farm and understood the value of plant breeding. At 21, entered priesthood and studied plant breeding in a monastery in the Czech Republic. Loved to read especially about natural sciences and was aware of Darwin’s findings. Studied the inheritance of traits in pea plants.

Gregor Mendel Studied traits that occur in distinct forms. Developed true-breeding varieties –When bred amongst themselves these plants produced offspring identical to the parent for that trait. Used mathematical analysis in his studies.

Definitions Gametes: reproductive cells produced by sexually reproducing organisms. –Two types: male gametes = sperm –In plants: contained in pollen Female gametes = eggs –In plants: contained in ovules –Ovules contained in carpels

Fertilization Fertilization: fusion of egg and sperm –Self-fertilized: fusion of sperm and egg from same plant –Cross fertilized: fusion of egg and sperm from two different plants Produced hybrids F 1 : first generation F 2 : second generation

True-breeding: -Plants allowed to self pollinate -Produced offspring identical to parents

Mendel’s Interpretations Modified to incorporate today’s vocabulary –Genes: the hereditary information that determines a single trait –Alleles: alternate forms of a gene Homozygous/purebred - When an organism inherits two of the same alleles for a trait. Heterozygous/hybrid - When an organism inherits two different alleles for one trait.

Heterozygous Alleles Homozygous Alleles

Why were all the seeds in the F 1 generation round? Dominant vs. Recessive Allele –Dominant: an allele that is expressed whenever it is present Symbolized by a upper case letter (R) –Recessive: an allele that is masked whenever the dominant allele is present. Symbolized by a lower case letter (r) Dominant and recessive alleles influence an organism’s phenotype

Genotype and Phenotype Genotype: Genetic makeup of an individual. It is determined by the alleles present for each trait. The type of genes an individual has. Phenotype: Physical appearance of a trait. It is the expression of the genotype.

Monohybrid Cross (One Factor Cross)

AA A A AA Father contributes: Mother contributes: or True Breeding

aa A A Aa Father contributes: Mother contributes: or Cross Breeding

Can you look at someone and determine their genotype? Testcross is used to determine the genotype of an individual –Testcross: crossing an organism with unknown genotype with one that is homozygous recessive for the trait. –Example: Plant that has round (R) seeds crossed with one that is homozygous recessive (rr) for wrinkled seeds R? x rr

Test Cross

Genetics (sec 11-3) Mendel performed experiments designed to follow 2 different genes in pea plants –Called a two factor cross (Dihybrid Cross)

Mendel’s Laws Based on Mendel’s data, he formulated two laws: –Law of Segregation : A parent contributes only one of its alleles for a trait to each offspring. –If parent is heterozygous for a trait, the particular allele donated to the offspring is random.

Law of Independent Assortment: Alleles of one gene are passed to offspring independently of the alleles of other genes. –Applies to the inheritance of two or more genes simultaneously.

Law of Independent Assortment Allows for new gene combinations or genetic recombination

Why Aren’t Members of the Same Species Identical? Sources of Genetic Variation –Law of Independent Assortment –Mutation –Crossing over

Mutations Definition = change in DNA sequence resulting the appearance of a new allele Can occur at any time, but not always beneficial Some can be harmful and result in death Some can result in the organism having traits that make them better suited to the environment

Do Mendel’s Laws Always Apply?

Incomplete dominance –Some alleles (traits) are neither dominant or recessive –Controlled by more than one allele The heterozygous phenotype is between the homozygous dominant or recessive. Both alleles are expressed.

Example of Incomplete Dominance

Shorthorn Cattle: Red male mated to a White female = Roan calf RR X rr = Rr Another example If a red bull (RR) is mated to a roan (Rr) cow, what color will the calves be?

Shorthorn: Red X Roan

What if Both Parents are Roan?

I.E. Incomplete dominance –Roan

Codominance Both alleles share dominance and are expressed I.E. speckled chickens –Black feathers dominant –White feathers dominant

Codominance Two allelesmultiple alleles heterozygous individualsTwo alleles are expressed (multiple alleles) in heterozygous individuals. Example: human blood typeExample: human blood type 1.type A= I A I A or I A i 2.type B= I B I B or I B i 3.type AB= I A I B 4.type O= ii

Polygenic Traits Interaction of several genes –Human skin color More than four genes control this trait –Human eye color –Human hair color

Recombination and crossing-over Recombination- offspring which have traits in a different combination from parents. Two kinds of recombination –Independent assortment of genes on different chromosomes –Crossing-over of genes on same chromosome

Crossing-Over (sec 11-4) Meiosis I –Starts with tetrads 4 chromatids (paired homologous chromosomes) –At this point crossing over can occur –Gives us even more variation in living organisms

Determination of Sex Drosophilia - fruit fly –XX - female –XY - male –XXY - female –XO - male Humans –XX - female –XY - male –XXY - male –XO - female Non-X or Y chromosomes are known as Autosomes

Sex linkage We have 2 sex chromosomes choices: –X and Y Females have XX chromosomes, males XY not YSex-linked genes located on X, not Y To show recessive trait, female needs 2 copies, male only needs 1.

Sex Linked Traits Color blindness (recessive) –Cannot distinguish between certain colors Most common form red-green colorblindness Hemophilia (recessive) –A protein needed for blood clotting is missing Duchenne Muscular Dystrophy (recessive) –Progressive weakening and loss of skeletal muscle