Patterns of Inheritance Chapter 9 Winter 2015 Aultman

Definitions - 1 Heredity is the transmission of traits from one generation to the next Genetics is the scientific study of heredity Gregor Mendel: 1860s; deduced the fundamental principles of genetics A character is a heritable feature that varies among individuals Seed color, pod shape, plant height A trait is a variant of a character (allele) Yellow or green seeds, wrinkled or inflated pods, dwarf or normal height Each of the characters Mendel studied occurred in two distinct traits

Mendel’s Experiments Garden peas created purebred varieties of plants Peas can self-fertilize crossed two different purebred varieties Hybrids are the offspring of two different purebred varieties The parental plants are the P generation. Their hybrid offspring are the F1 generation. A cross of the F1 plants forms the F2 generation A monohybrid cross is a cross between purebred parent plants that differ in only one character

Definitions - 2 A gene locus is a specific location of a gene along a chromosome Alleles alternative versions of genes For each inherited character, an organism inherits two alleles, one from each parent An organism is homozygous for that gene if both alleles are identical. An organism is heterozygous for that gene if the alleles are different. If two alleles of an inherited pair differ, then one determines the organism’s appearance and is called the dominant allele and the other has no noticeable effect on the organism’s appearance and is called the recessive allele. Dominant alleles determine the plant’s phenotype

Mendel’s Law of Segregation Figure 9.6, page 149 Gametes carry only one allele for each inherited character The two alleles for a character segregate (separate) from each other during the production of gametes. This statement is called the law of segregation Punnett Square Ratio of offspring characteristics reflects composition of parental traits

Mendel’s Law of Independent Assortment dihybrid cross is the mating of parental varieties differing in two traits each pair of alleles segregates independently of the other pairs during gamete formation Offspring have an equal chance of any combination Figure 9.8 page 150 A testcross is a mating between an individual of dominant phenotype (but unknown genotype) and a homozygous recessive individual Predict the outcome:

The Rules of Probability Mendel’s strong background in mathematics helped him understand patterns of inheritance The rule of multiplication states that the probability of a compound event is the product of the separate probabilities of the independent events Figure 9.1. page 154 For a monohybrid cross, each cell of the Punnett square has a 25% probability; each cell – one combination of genes Homozygous dominant = 25% Homozygous recessive – 25% Heterozygous = 25% + 25% = 50%

Single-Gene Human Disorders Many human traits show simple inheritance patterns and are controlled by single genes on autosomes. Most human genetic disorders are recessive Individuals who have the recessive allele but appear normal are carriers of the disorder Cystic fibrosis: recessive allele carried by about one in 31 Americans Achondroplasia is a form of dwarfism. The homozygous dominant genotype causes death of the embryo only heterozygotes have this disorder

Incomplete Dominance in Plants and People F1 hybrids have an appearance between the phenotypes of the two parents hypercholesterolemia, heterozygotes have blood cholesterol levels about twice normal, and homozygotes have about five times the normal amount of blood cholesterol and may have heart attacks as early as age 2. Polygenic inheritance is the additive effects of two or more genes on a single phenotype

Co-Dominance: ABO Blood Groups The immune system produces blood proteins called antibodies that bind specifically to foreign carbohydrates A person whose blood has A or B carbohydrates does not make antibodies to them; antibodies are only made to foreign antigens. If a donor’s blood cells have a carbohydrate (A or B) that is foreign to the recipient, the donated blood cells may be clumped together by the recipient’s blood, potentially killing the recipient The clumping reaction is the basis of a blood-typing lab test The human blood type alleles IA and IB are codominant, meaning that both alleles are expressed in heterozygous individuals who have type AB blood

Pleiotropy and Sickle-Cell Disease one gene influences several characters Sickle-cell disease results in abnormal hemoglobin proteins, and causes disk-shaped red blood cells to deform into a sickle shape with jagged edges

Chromosome theory of inheritance genes are located at specific positions (loci) on chromosomes and the behavior of chromosomes during meiosis and fertilization accounts for inheritance patterns. chromosomes undergo segregation and independent assortment during meiosis and account for Mendel’s laws

Linked Genes are located close together on a chromosome and tend to be inherited together Thomas Hunt Morgan used the fruit fly Drosophila melanogaster and determined that some genes were linked based on the inheritance patterns of their traits.

Genetic Recombination: Crossing Over Crossing over can separate linked alleles, produce gametes with recombinant gametes, and produce offspring with recombinant phenotypes. The percentage of recombinant offspring among the total is called the recombination frequency Linkage Maps method for mapping the relative gene locations Figure 9.28, page 164

Sex Chromosomes and Sex-Linked Genes Sex chromosomes influence Gender: XX or XY Other traits that are sex-linked Red-green colorblindness caused by a malfunction of light-sensitive cells in the eyes Hemophilia sex-linked recessive blood-clotting trait that may result in excessive bleeding and death after relatively minor cuts and bruises Figure 9.31 page 166