sign in sign up
<<
Home , Dominance (genetics)

Genetics

Mendel, Human , & The Idea Genetics . Genetics -the study of inheritance. -how traits (characteristics) are passed down from to offspring.

. - mid 1800’s-Austrian monk -founded modern genetics with data gathered from growing . . Peas -easy to grow and had many traits. . Self-pollinate and cross-pollinate* Flower Anatomy

Pollen is here

Eggs are here Pollen grains and eggs

Pollen is flower Ovules are eggs at the flower base Mendel’s Terms

Trait (variant for a character, i.e., brown)

True-breeding (all offspring of same variety) -Pure-

Hybrid (offspring of 2 different true-)

Alleles- different forms of gene P generation ()

F1 generation (first generation)

F2 generation (second generation)

Mendel’s Experiments:

Self and Cross Pollinated

1. Focused on -garden traits by crossing different traits one at a time: Flower/ coat color: gray vs white Seed color: yellow vs. green Seed shape: smooth vs. wrinkled Pod color: green vs. yellow Pod shape: inflated vs. pinched Stem height: tall vs. short Flower position: axial vs. terminal

2. Counted offspring of each trait form and analyzed the results using math. Vocabulary . –forms of a gene.

. Homozygous: pair of identical alleles for a trait . (pure/true-bred) . Heterozygous: two different alleles for a trait . () . : an organism’s observed trait . : an organism’s genetic makeup Law of Segregation . Different alleles account for variations in inherited characteristics

. For a trait-an organism inherits 2 alleles, one from each parent

. Alleles - segregate (separate) during production (). Probability and Punnett squares

Punnett square- a tool to calculate expected of offspring of a cross. Principle of Dominance

Heterozygous offspring, (Aa) . Dominant - fully expressed in the organism’s phenotype

. Recessive allele, has no effect on the organism’s phenotype

. (* 1 pattern of inheritance)

Law of Independent Assortment

. Alleles on different , segregate independently of each other=

. Mendel’s Law of Independent Assortment Dihybrid

F1 generation

Two traits- Two different chromosomes

2 homozygous Parents with different traits produce Dihybrid offspring :

F2 generation

Phenotype Ratio:

9:3:3:1 Additional Genetic Patterns Mendel’s peas Other Patterns Complete Incomplete Dominance Dominance Codominance Lethal Alleles

Two alleles /gene Multiple Alleles

One gene affects …Many characteristics one trait () Two (or more) affect one trait (Polygenic Traits) One gene masking another() Other Inheritance Patterns

. Incomplete dominance - appearance between the of the 2 parents. Ex: snapdragons

CR=red CW=white Pink- heterozygous CRCW Recognize- Different allele representation Incomplete Dominance Problem

Snapdragon problem: Use the allele representation C R = Red, C W = white The outcome of a cross between a red- flowered plant and a pink-flowered plant? P = C R C R x C R C W

50% red, C R C R 50% pink, C R C W

Sometimes Genotype Written: RR, RW Incomplete Dominance

Incomplete dominance: neither allele masks the other and both are observed as a blending in the heterozygote

RR x R’R’ Four o’clock flowers Red White R = red, R’ = white

RR’ pink Incomplete Dominance

RR’ x RR’ Pink x Pink

Genotypic Ratio: Phenotypic Ratio: Codominance

. Codominance: two alleles equally expressed, but not blended, like incomplete dominance . Ex: coat- cows- both red and white hairs Cross a red cow with a white bull. What is the expected genotype and phenotypes of the offspring?

P = RR x WW

RW, roan Multiple Alleles -Having more than 2 alleles for a trait.

Human Blood alleles- IA ,IB, i codominance & multiple IA = IB > i IA and IB are codominant. IA and IB are completely dominant over i. Antigens on Red Blood Cells

IAi IBi

IAIB Lethal Alleles

Gene capable of causing death- usually in embryo.

Example: ML = tailless, lethal in homozygote m = tail

Tailless male x Tailless female Polygenic Traits

. Most of your traits are controlled by the interaction of many genes.

. Multiple genes working together produce a continuous distribution in a “Bell Shape” curve of degrees. Examples of Polygenic Traits

Individual genes of a . Body Type polygenic trait follow . Height Mendel's laws but . Skin Color together do not . Hair color produce Mendelian . ratios. . Intelligence . We often see the famous “Bell Curve”

Example of Polygenic Inheritance Two genes affecting skin coloration

Number of Skin Color* Genotypes % Pigmentation Dominant (Phenotype) Alleles 0 White aabb 0-11%

1 Light Black Aabb or aaBb 12-25%

2 Medium Black AAbb or AaBb or 26-40% aaBB 3 Dark Black AABb or AaBB 41-55%

4 Darkest Black AABB 56-78%

*Based on a study conducted in Jamaica. Polygenic Inheritance

Medium Black Woman X Darkest Black Man (mother is white)

Pleiotropic Effects

One gene affects many phenotypic characteristics

Allele S S’ Gene Product A Hemoglobin S Cell Shape Round Sickled under

low O2 tension Response to Susceptible Resistant in SS’ Malaria genotype Pleiotropy

Pleiotropy refers to an allele which has more than one effect on the phenotype.

This can be seen in human diseases such as or sickle cell . In these diseases, multiple symptoms can be traced back to one defective allele.

Recent studies show … . Hypertension . . Cancers . Allergies . Cardiovascular diseases . Behavioral traits (alcoholism and phobias)

…..have some genetic link but also environmental explanation. Environmental Effects Expression of some genes may be impacted by environment Gene for pigment production expressed in cooler regions of body Another example of environmental influence: . Hydrangeas – same genotype, different environments  different color flowers

Acid pH Alkaline pH  Epistasis

. An allele of one gene masks the expression of alleles of another gene and expresses its own phenotype instead. . Gene that masks = epistatic gene . Gene that is masked = hypostatic gene Epistasis . Action of genes at one location modify expression genes at another location .Effects often complex . Examples . Coat color in Labrador dogs . Snake coloration Epistasis in Labrador Dogs

. Bb or BB  dark (black) pigment produced . bb  light (brown) pigment produced

. Ee or EE  deposition of . ee  deposition of pigment blocked Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

ee E_ No dark pigment in fur Dark pigment in fur Yellow Lab

E_bb E_B_ eebb eeB_ Chocolate Lab Black Lab Yellow fur; Yellow fur; Brown fur, Black fur, brown nose, black nose, nose, lips, nose, lips, lips, eye rims lips, eye rims eye rims eye rims Interacting Genes Affecting a Single Characteristic eg. Skin coloration in snakes

One gene O = orange pigment o = no orange pigment

Second gene B = black pigment b = no black pigment Interacting Genes Affecting a Single Characteristic

eg. Skin coloration in snakes Oo Bb x Oo Bb OB Ob o B o b OB OO BB OOBb Oo BB Oo Bb Ob OO Bb OO bb Oo Bb Oo bb o B Oo BB Oo Bb o o BB o o Bb o b Oo Bb Oo b b o o Bb o o b b Interacting Genes Affecting a Single Characteristic eg. Skin coloration in snakes

OoBb x OoBb

9/16 O_B_ 3/16 O_bb 3/16 ooB_ 1/16 oobb Ch 14 Gene Disorders – Gene

. Gene disorder -the harmful effect a mutated allele produces when it occurs with significant frequency in a population.

. Can be inherited in several ways Dominant/ recessive Codominance Recessive Disorders

Ex  PKU -  CF -Cystic fibrosis  Tay-Sachs  Prader-Willi Gene Disorders

Phenylketonuria (PKU) A recessive disorder that occurs in 1 of 10,000 people. A defective gene on 12 is responsible. Cannot break down -Amino acid . Recessive Disorders Cystic Fibrosis A recessive disorder - mutated gene on chromosome 17. Excessive secretion of the mucus in the body.

Tay-sachs Disease A recessive disorder that occurs in 1 of 5,000 people - European Ashkenazi Jews Defective gene on chromosome 15. Excess lipids in brain Prader Willi Syndrome A recessive disorder that occurs in 1 of 15,000 people. Determined by a set of Mutated genes on chromosome 15. Dominant Disorders

Marfan’s Syndrome Huntington’s disease  (NF) Huntington Disease A dominant disorder that occurs in about 1 of 10,000 people. A dominant gene on is responsible. It causes degeneration of neurons producing dementia, and random jerking movements. Other Patterns of Inheritance Multiple Alleles: ABO Blood Group

. Codominance - No single allele is dominant, and each allele has its own effect. . Blood alleles A and B

Recessive . Blood O allele Human Blood Types

Phenotype Genotype O i recessive

A IA IA or IAi

B IB IB or IB i

AB IAIB Inherited traits and disorders can be… . Autosomal – non-sex chromosomes(1-22) ex sickle cell, CF, PKU

. Sex-Linked – sex chromosomes (23rd)

. A sex-linked trait is expressed by a gene on the sex chromosomes . We study traits on the . Human Chromosomes

. 22 of the pairs are called and are numbered from largest to smallest.

. The 23rd pair are the sex chromosomes: . XX females . XY males Chromosomes Sex-Linked Gene Disorders

1- Color Blindness Recessive disorder. Occurs in 1 of 10 males, green –red most frequent

2- Hemophilia A and B Recessive disorders that affect 1 of 5,000 males. Interferes with normal blood clotting and occur at positions on a chromosome on the X chromosome. Sex Linked

Genetics problems -must include the chromosomes when doing problems. X Y The allele is superscripted on X

H h X Y or X Y H=normal h=hemo Pedigrees

Pedigrees - used to study inheritance of trait over generations Ex. hemophilia - inherited condition where blood is slow to clot or does not clot at all

. Royal hemophilia - sex-linked

You must learn how to analyze a pedigree! Make sure you understand the symbols and shading as we study the pedigrees on your problem sheet. Royal Hemophilia Pedigree √ for Understanding - Pedigree What is the pattern of inheritance? Autosomal Dominant What disorder?

Huntington’s Disease Human Chromosomes

. One X chromosome in females is inactivated early in embryonic development.

. Visible as a darkly staining Barr body attached to the nuclear membrane.

Chromosomes can be arranged -match homologous sister chromatids - Karyotype

Human male karyotype Karyotypes of human chromosomes Nondisjunction

Can cause of chromosomal disorders Failure of sister chromatids to separate in meiosis

Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.

TRISOMY 13

Patau syndrome , cleft lip & palate, , low-set ears Many die shortly after birth.

Only 5-10 % live past first year. Trisomy -18- Edwards Syndrome

Abnormalities in many body parts. Many die at birth Or short after. Nondisjunction of Sex Chromosome Nondisjunction

Klinefelter Syndrome (XXY, XXXY, XXXXY) Occurs in about 1 of 1,000 males. Caused by an extra X chromosome Turner’s Syndrome (XO) Occurs in about 1 of 10,000 females. These females have immature female appearance, do not develop secondary sex characteristics, and some lack internal reproductive organs.

2n=45