Patterns of Inheritance

Lecture on Inheritance

CC-BY, Biology, Leslie Orzetti-Gollhofer

1. Patterns of Inheritance

2. Chromosome Review

3. Genetics Study of the patterns of inheritance Mendelian Genetics Gregor Mendel Pea plant experiments Grow easily Distinguishable characteristics Round/Wrinkly, Yellow/Green, Tall/Short Can control mating

4. Terminology Allele – Alternate forms of a gene Dominant Allele – trait that exerts its effects whenever present Symbolized by capitol letter (B) Ex. Dwarfism, Huntington’s disease Recessive Allele – trait that is masked if a dominant trait is present Symbolized by lower case letter (b) Ex. Blue eyes

5. Terminology Genotype – Genetic makeup of an organisms Homozygous – 2 alleles that are the same (BB or bb) Heterozygous – 2 different alleles (Bb) Phenotype – Observable characteristics Brown eyes, brown hair, tall, short Generation – group of organisms born around the same time P Generation – Parental F1 – offspring of parent, first filial generation F2 – 1 st generation of offspring

6. Genetic Terminology Term Definition Generations P First (parental) generation F 1 children (Latin filii ) of generation P F 2 children of generation F 1 Chromosomes and Genes Chromosome macromolecules found in cells, consisting of DNA, protein and RNA Gene unit of heredity of a living organism Locus specific location of a gene, DNA sequence, or position on a chromosome Allele one of a number of alternative forms of the same gene or genetic locus Term Definition Dominant and recessive Dominant Allele allele that is always expressed if present Recessive Allele allele that is only expressed if the dominant allele is not present Genotypes and Phenotypes Genotype genetic makeup of a cell, an organism, or an individual Homozygous contains two of the same allele of a gene Heterozygous contains two different alleles of a gene Phenotype organism's observable characteristics or traits True Breeding pure-bred, is an organism that always passes down a certain phenotypic trait to its offspring Wild Type most common phenotype in nature Mmutant physical or genetic difference from a mutation in a gene

7. Tracking Inheritance Punnett Square – diagram showing genotypes in a test cross Test cross – a Punnett square done to figure out the genotype of an unknown Always use a homozygous recessive individual with unknown Monohybrid Cross – Mating between individuals that are BOTH heterozygous Dihybrid Cross – Mating between individuals that are heterozygous for 2 traits (genes )

8. Genotypic ratio 1:2:1 (1 BB: 2 BB: 1 bb) Phenotypic ratio 3:1 (3 pink: 1 white) Monohybrid Cross

9. Dihybrid Cross Phenotypic ratio 9:3:3:1 (9 Smooth brown, 3 Smooth white 3 Curly brown 1 Curly white)

10. Laws of Segregation and Independent Assortment Segregation - Two alleles of each gene are packed into separate gametes Alleles move apart during meiosis Occurs during meiosis 1 – Metaphase Homologous chromosome move to opposite ends of Poles Assortment - The segregation of alleles of one gene does not influence the alleles for another gene Alleles are RANDOMLY packaged in gametes Use Product Rule – The chance that 2 independent events will both occur = the product of individual chances that each event will occur

11. Laws of Segregation and Assortment

12. Product Rule

13. Genetics Problem The ability to curl your tongue up on the sides (T, tongue rolling) is dominant to not being able to roll your tongue. A woman who can roll her tongue marries a man who cannot. Their first child has his father’s phenotype. What are the genotypes of the mother, father, and child? What is the probability that their second child will be a tongue roller?

14. About 70% of Americans perceive a bitter taste from the chemical phenylthiocarbamide (PTC). The ability to taste this chemical results from a dominant allele (T) and not being able to taste PTC is the result of having two recessive alleles (t). Albinism is also a single locus trait with normal pigment being dominant (A) and the lack of pigment being recessive (a). A normally pigmented woman who cannot taste PTC has a father who is an albino taster. She marries a homozygous, normally pigmented man who is a taster but who has a mother that does not taste PTC. What percentage of the children will be albinos? What percentage of the children will be non-taster of PTC? Genetics Problem

15. Recessive Disorders Recessive Disorder – disorder is ONLY expressed when both alleles are recessive Can be “carried” Heterozygous individuals c an carry gene but not e xpress disease A a A AA Aa a Aa aa Sickle Cell Anemia Genotype Phenotype AA Normal Aa Carrier aa Sickle Cell

16. Recessive Disorders Sickle Cell Anemia

17. Dominant Disorders Dominant Disorder Condition is expressed when an individual has at least one dominant allele A a A AA Aa a Aa aa

18. Autosomal Disorders Disorder Likelihood Major Symptoms Recessive Disorders Albinism 1/22,000 Lack of pigment in skin. appear white or very pale. Skin can burn more easily from overexposure. Cystic Fibrosis 1/2500 ( Caucasions ) Excess mucus in lungs, digestive tracts, liver; increased susceptibility to infections, death in early childhood if not treated Galactosemia 1/60,000 (Caucasians) Galactitol accumulates in body tissues. R esults in an enlarged liver, cirrhosis, renal failure, cataracts, brain damage, and ovarian failure. Mortality in infants is about 75%. Phenylketonuria (PKU) 1/15,000 (US) Abnormally high levels of phenylalanine accumulate in blood, toxic to the brain. Intellectual disability, brain function abnormalities, mood disorders, irregular motor functioning, and behavioral problems. Sickle-cell disease 1/5,000 (US) May lead to tissue damage. Complications include risk of infection, stroke, and chronic pain. Tay –Sachs disease 1 /3,500 (Ashkenazi Jews) Typically noticed in infants around 6 months, display abnormally strong response to sudden noises or other stimulus, known as the “startle response.” There may also be listlessness or muscle stiffness (hypertonia). Dominant Disorders Alzheimer’s disease unknown Dementia ; memory loss, mental decay; not all cases are result of genetics Huntington’s disease 5–10 /100,000 Neurodegenerative genetic disorder, affects muscle coordination, leads to cognitive decline, behavioral symptoms. Hypercholesterolemia unknown Presence of high levels of cholesterol in blood ; can lead to atherosclerosis and other heart problems; can be caused by genetics or lifestyle

19. Variations on Mendel’s Laws Incomplete Dominance Individual who is heterozygous show intermediated phenotype

20. Variations on Mendel’s Laws Codominance Two different alleles are fully expressed in phenotype

21. Variations on Mendel’s Laws Pleiotropy O ne gene has many effects Sickle Cell Allele Abnormal Protein Abnormal Shaped Red Blood Cell Organ Damage Kidney Failure Heart Failure Spleen Damage Brain Damage Other Effects Pain and Fever Joint Problems Physical Weakness Anemia Pneumonia

22. Variations on Mendel’s Laws Polygenic Inheritance – Many genes control one phenotype Eye color, hair color, height, skin color

23. Sex Linked Genes Sex Determination Female – XX Male – XY Can be carried on either X or Y chromosome More common on X chromosome Affect more males than females

24. X A X a X A X A X A X A X a Y X A Y X a Y

25. X-Linked Disorders Disorder Genetics Characteristics X-Linked Recessive Inheritance Duchenne muscular dystrophy Mutation in dystrophin gene Early life muscle degeneration; eventual death Fragile X syndrome Expansion of CGG trinucleotide repeat on X chromosome Most widespread single-gene cause of autism and inherited cause of intellectual disability, especially among boys Hemophilia A Mutant allele; ~70% of the time X-linked recessive trait, ~ 30% of cases arise from mutations Deficiency in clotting factor VIII, causes increased bleeding; usually affects males Red–green color blindness Absence or mutation of genes for red or green color receptors Difficulty discriminating red and green hues Rett syndrome Mutations in gene MECP2 located on X chromosome Typically no verbal skills, 50 % of individuals affected do not walk. Scoliosis, growth failure, constipation very common X-Linked Dominant Inheritance Hypertrichosis Unknown Abnormal amount of hair growth over body X-linked hypophosphatemia Mutation in PHEX gene sequence ( Xp.22); subsequent inactivity of PHEX protein Can cause bone deformity including short stature and genu varum (bow leggedness). Retinitis pigmentosa (some forms) Unknown Degenerative eye disease; causes severe vision impairment; often blindness