Patterns of Inheritance Chapter 9

Gregor Mendel Deduced the fundamental principles of genetics Cross-fertilization

What’s with the Peas? Little spontaneous variation between generations Can self-fertilize Easy to control pollination Possessed several easily observable traits Pea form Pea color Flower location Flower color Stem size

What do we get?? Genetic cross P generation F1 generation Bb x Bb P generation F1 generation F2 generation

Monohybrid Cross Cross between parent plants that differ in only one characteristic Mendel developed four hypotheses from the monohybrid cross: There are alternative forms of genes Alleles For each characteristic, an organism inherits two alleles One from each parent Alleles can be dominant or recessive Gametes carry only one allele for each inherited characteristic

Mendel’s Laws Genes Set of instructions that determine characteristics of an organism Segments of nucleic acid that specifies a trait Found at designated place on chromosomes Locus Not all copies of a gene are identical

Mendel’s Laws Alternative forms of a gene lead to the alternative form of a trait Alleles way of identifying the two members of a gene pair which produce opposite contrasting phenotypes Chromosomes that are homologous are members of a pair and carry genes for the same traits in the same order

Genes v. alleles Genes Alleles Basic instruction Sequence of DNA General Hair color Alleles Variations of that instruction Specifics Brown hair

Alternative alleles of genes are located on homologous chromosomes

Genotype verse Phenotype the alleles an individual receives at fertilization Homozygous an organism has two identical alleles at a gene locus Heterozygous an organism has two different alleles at a gene locus Phenotype the physical appearance of the individual

The journey from DNA to phenotype

Mendel Proposes a Theory genetic traits are assigned a letter symbol referring to their more common form Dominant traits are capitalized while a lower-case letter is reserved for the recessive trait P signifies purple p signifies white

Describing Genotypes Homozygous Dominant when both alleles are dominant BB Homozygous Recessive when both alleles are recessive bb Heterozygous when one allele is dominant and one is recessive Bb

Punnet Square….. Genetic cross determines arrangement

Variations on Mendel’s Laws Incomplete dominance Multiple allelism & codominance Pleiotropy Polygenic inheritance Environmental factors

Why Some Traits Don’t Show Mendelian Inheritance Incomplete dominance Alleles have combined (equal) effect on phenotype of heterozygote Phenotype is intermediate

Multiple Allelism: existence of more than 2 alleles of gene Example: Blood type (A, B, O) Remember: Each person still only has 2 alleles for that trait, but more than 2 exist

Multiple Allelism: Blood typing ABO Blood Type in Humans exhibits multiple allelism Phenotype Genotype O OO A AA or AO B BB or BO AB AB * How many ALLELES are there? 3 ( A, B, O) How many Phenotypes are there? 4 (A, B, AB, O) How many Genotypes are there? 6

Question: If a woman with blood type O marries a man with blood type B, can they have a child with blood type A? Phenotype Genotype O OO A AA or AO B BB or BO AB AB* No. The mother’s genotype must be OO and the father’s either BB or BO. Their child will either be type B (BO) or type O (OO)

Why Some Traits Don’t Show Mendelian Inheritance Codominance A gene may have more than two alleles in a population in heterozygotes, there is not a dominant allele Both alleles are expressed Relationship between the A & B alleles in blood typing

Why Some Traits Don’t Show Mendelian Inheritance Pleiotropic effects Allele that has more than one effect on a phenotype these effects are characteristic of many inherited disorders Sickle-cell anemia Must be homozygous for sickle cell allele

Why Some Traits Don’t Show Mendelian Inheritance Continuous variation Characters can show a range of small differences when multiple genes act jointly to influence a character Polygenic

Phenotypes are not always a direct translation of genotype Phenotypes may also be influenced by the environment Examples? skin color influenced by sun height/weight influenced by nutrition animal coat influenced by climate

Remember….. P = G + E

Sex-linked Traits Female XX Male XY Genes located on the X or Y chromosome are sex-linked X and Y chromosomes are not homologous, they contain different genes

Sex-linked traits Sex chromosomes Sex-linked genes Are designated X and Y Determine an individual’s sex Influence the inheritance of certain traits Sex-linked genes Are any genes located on a sex chromosome

Sex-Linked Traits Females (XX) have 2 copies of each gene on the X chromosome Males (XY) have only 1 copy of each gene on the X chromosome Females can show a dominant condition if present on 1 or both X chromosomes Females can only show a recessive condition if present on both X chromosomes Males ALWAYS show X-linked alleles, regardless of dominance

Sex-Linked Disorders in Humans number of human conditions result from sex-linked (X-linked) genes Red-green color blindness characterized by a malfunction of light-sensitive cells in the eyes

Question….. Will a mother that is colorblind automatically have a son that is colorblind?

Mutations Changes to the nucleotide sequence of the genetic material of an organism Can be caused by: copying errors in the genetic material during cell division exposure to UV light or chemical mutagens Viruses can be induced by the organism itself Create variety within gene pool Less favorable verse more favorable

Human Heredity To study human heredity, scientists examine crosses that have already been made Pedigree Determine whether a trait is sex-linked or autosomal and whether the trait’s phenotype is dominant or recessive

Huntington’s disease is a dominant genetic disorder

Recessive Disorders Most human genetic disorders are recessive Individuals can be carriers of these diseases