Patterns of Inheritance. Game Plan. Gregor Mendel ( ) Overview of patterns of inheritance Determine how some genetic disorders are inherited

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1 Patterns of Inheritance Game Plan Overview of patterns of inheritance Determine how some genetic disorders are inherited Gregor Mendel (8-88) Austrian monk responsible for developing the modern idea of how traits were inherited. Mendel showed that parents pass heritable traits to offspring (factors now called genes).

2 Mendel s s four hypotheses Alternative versions of genes now called alleles. An organism inherits two alleles, one from each parent. One allele determines the organisms phenotype (usually!) Genetic mechanisms of genetic disorders Acondroplasia (dwarfism) is a phenotype resulting from an autosomal dominant allele people with darfism have normal copy (a) and one abnormal copy of the gene (A) two copies of the abnormal gene (AA) is generally fatal at or shortly after birth Osteogenesis Imperfecta (OI) also results from an autosomal dominant allele PKU (phenylketonuria) is an autosomal recessive disorder DMD (Duchenne s Muscular Dystrophy) is a recessive X- linked condition The role of sex chromosomes in genetics Many animals, like humans, have a pair of sex chromosomes, which determine sex. In humans, genes on the sex chromosomes have a unique pattern of inheritance. Sex chromosomes also contain genes that influence other traits, not just maleness or femaleness.

3 (male) + XY Parents diploid cells (female) + XX Sperm Egg +X +Y +X + XX Offspring (diploid) + XY What does this mean for sex-linked genes? Sex-linked genetic disorders affect mostly males. Males will express (have a phenotype for) X-linked disorders when one recessive allele is present. Heterozygous females will not express X-linked disorders when one recessive allele is present. X Y 3

4 Sex Linked Trait Question: Colorblindness is due to the X-linked recessive allele b, while the X-linked dominant allele B leads to full color vision. Predict the ratio of offspring phenotypes for each of the following: Female who had a color blind father x male with normal vision Other variations on Mendel s laws Incomplete dominance results in intermediate phenotypes. P generation Red RR White rr Gametes R r F generation Pink Rr Gametes R r Sperm R r F generation R RR rr Eggs r Rr rr

5 Other variations on Mendel s laws Incomplete dominance results in intermediate phenotypes. Some traits have more than one allele Variations on Mendel s laws Many genes have more than two alleles, or multiple alleles in a population (i.e. multiple versions of the trait in the population). Individuals still carry only two alleles, but more than two possible alleles exist in a population. For example, the ABO blood group in humans involves three alleles (O,A,B) of a single gene, accounting for phenotypes. Blood Group (Phenotype) Genotypes Red Blood Cells O ii A I A I A or I A i Carbohydrate A B I B I B or I B i Carbohydrate B AB I A I B A and B are co-dominant. 5

6 Other variations on Mendel s laws Incomplete dominance results in intermediate phenotypes. Some traits have more than one allele Pleiotropy: Some alleles affect more than one trait Variations on Mendel s laws A single gene might influence many phenotypes. This is called pleiotrophy. For example, sickle-cell anemia in humans. Sickle-cell is controlled by one gene with two alleles. But, having two recessive alleles has impacts on many traits! Individual homozygous for sickle-cell allele Sickle-cell (abnormal) hemoglobin Abnormal hemoglobin crystallizes, causing red blood cells to become sickle-shaped Sickle cells Breakdown of red blood cells Clumping of cells and clogging of small blood vessels Accumulation of sickled cells in spleen Physical weakness Anemia Heart failure Pain and fever Brain damage Damage to other organs Spleen damage Impaired mental function Paralysis Pneumonia and other infections Rheumatism Kidney failure 6

7 Back to Mendel s five hypotheses Alternative versions of genes now called alleles. An organism inherits two alleles, one from each parent. One allele determines the organisms phenotype (usually!). Law of segregation: A sperm or egg carries only one allele for each inherited character because allele pairs segregate from each other during the formation of gametes. Law of independent assortment: To understand independent assortment we will track two traits at a time.. Let s follow two different traits: Seed shape (R round, r wrinkled). Seed color (Y yellow, y green). This cross is termed a dihybrid cross. Heritable traits FLOWER COLOR Purple White P p A a FLOWER POSITION SEED COLOR SEED SHAPE POD SHAPE POD COLOR Axial Yellow Round Inflated Green Terminal Green Wrinkled Constricted Yellow Y R I G y r i g T t STEM LENGTH Tall Dwarf 7

8 Hypothesis: Dependent assortment Hypothesis: Independent assortment P generation RRYY rryy RRYY rryy Gametes RY ry Gametes RY ry F generation RrYy RrYy RY Sperm ry Sperm RY ry Ry ry F generation RY RY RRYY RrYY RRYy RrYy Eggs ry Hypothesized (not actually seen) ry Eggs Ry ry RrYY rryy RrYy rryy RRYy RrYy RRyy Rryy RrYy rryy Rryy rryy Actual results (support hypothesis) Yellow round Green round Yellow wrinkled Green wrinkled Mendal s Laws reflect rules of probability F genotypes Bb male Formation of sperm Bb female Formation of eggs B b B B B B b b b B b b F genotypes Interpretation of dihybrid crosses The data supported a hypothesis that each pair of alleles segregates independently of other pairs of alleles. This is Mendel s law of independent assortment. 8

9 Mendel s laws correlate with homologous chromosome behavior during meiosis: The law of segregation depends on separation of homologous chromosomes in anaphase of meiosis I. The law of independent assortment depends on genes being on different chromosomes and crossing-over in metaphase I of meiosis I. Mendel s s five hypotheses Alternative versions of genes now called alleles. An organism inherits two alleles, one from each parent. One allele determines the organisms phenotype (usually!) Law of segregation: A sperm or egg carries only one allele for each inherited character because allele pairs segregate from each other during the formation of gametes. Law of independent assortment: pair of alleles segregate independently of other pairs of alleles Dihybrid Cross Question Complete a punnet square for a dihybrid cross of a female with blue eyes (bb) who does not an allele for PKU (PP) and a male from a family with only brown eyes (BB) and who has PKU (pp). 9

10 Complications with the law of independent assortment Genes on the same chromosome tend to be inherited together. They are linked genes. This is an exception to the law of independent assortment. Experiment Purple flower PpLl PpLl Long pollen Phenotypes Purple long Purple round Red long Red round Observed offspring 8 55 Prediction Phenotypic ratio is not as expected! Explanation: linked genes Parental diploid cell PpLl PL pl Meiosis Most gametes PL pl Fertilization PL Most offspring Eggs pl Sperm PL pl PL PL PL pl pl pl PL pl 3 purple long : red round Not accounted for: purple round and red long 0

11 Crossing over produces new combinations of alleles Linked alleles can be separated by crossing over Recombinant chromosomes are formed in meosis I But genes that are close together are more likely to be linked Geneticists measure genetic distance by recombination frequency (centimorgan) Now we also have physical maps of base pairs A B A B A b a b a B a b Tetrad Crossing over Gametes Experiment Gray body, long wings (wild type) GgLl Female Black body, vestigial wings ggll Male Offspring Gray long Black vestigial Gray vestigial Black long Parental phenotypes Recombinant phenotypes Recombination frequency = 39 recombinants = 0.7 or 7%,300 total offspring

12 Explanation GgLl (female) G L ggll (male) G L G l g L Eggs Sperm G L G l g L Offspring