What happens to all those genes Genetic inheritance

What happens to all those genes? Genetic inheritance Copyright © 2009 Pearson Education, Inc.

Let’s look at an interesting example… the hybrid Hybrid: the offspring of two different varieties Cross a female tigress (Panthera tigress) with a male lion (Panthera leo) and get a LIGER Males are sterile, females “can” reproduce VERY large; have lived to 24 years but often die shortly after birth Copyright © 2009 Pearson Education, Inc.

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What determines what the liger will look like, if it will live, if it can reproduce…? Copyright © 2009 Pearson Education, Inc.

What determines what the liger will look like, if it will live, if it can reproduce…? Genes Copyright © 2009 Pearson Education, Inc.

Mendel’s Laws: Heritable factors called genes dictate offspring’s characteristics Gregor Mendel discovered genetics using the garden pea Genes: Heritable factors passed from parent to offpsring Advantages of using pea plants Controlled matings Self-fertilization or cross-fertilization Observable characteristics with two distinct traits Copyright © 2009 Pearson Education, Inc.

Petal Stamen Carpel

White 1 Removed stamens from purple flower Stamens Carpel Parents (P) 2 Purple 3 Transferred pollen from stamens of white flower to carpel of purple flower Pollinated carpel matured into pod 4 Offspring (F 1) Planted seeds from pod

Character Flower color Traits Purple White Axial Terminal Seed color Yellow Green Seed shape Round Wrinkled Pod shape Inflated Constricted Pod color Green Yellow Tall Dwarf Flower position Stem length

Mendel’s law of segregation: the inheritance of a single character P generation (true-breeding parents) Purple flowers White flowers F 1 generation All plants have purple flowers Fertilization among F 1 plants (F 1 ´ F 1) F 2 generation 3 – 4 Copyright © 2009 Pearson Education, Inc. of plants have purple flowers 1 – 4 of plants have white flowers

Mendel’s law of segregation A monohybrid cross Track ONE character Parental (P) generation: purple flowers white flowers F 1 generation: all plants with purple flowers F 2 generation: ¾ of plants with purple flowers ¼ of plants with white flowers Questions? Why did one trait seemed to disappear in the F 1 generation? Why that trait reappeared in one quarter of the F 2 offspring? Copyright © 2009 Pearson Education, Inc.

Mendel’s law of segregation Here is why (4 hypotheses): 1. Genes are found in alternative versions called alleles (all alleles/genes found in an organism are called the genotype) 2. For each characteristic, an organism inherits two alleles; the alleles can be the same or different A homozygous genotype has identical alleles A heterozygous genotype has two different alleles Copyright © 2009 Pearson Education, Inc.

Mendel’s law of segregation Here is why (4 hypotheses): 3. If alleles are different: - The dominant allele determines the organism’s appearance (phenotype) - The recessive allele has no noticeable effect 4. Law of segregation: Allele pairs separate (segregate) from each other during the production of gametes so that a sperm or egg carries only one allele for each gene How do we use these to explain Mendel’s experiment? ? Copyright © 2009 Pearson Education, Inc.

Genetic makeup (alleles)� pp PP P plants Gametes All p All P F 1 plants (hybrids) All Pp Gametes 1 – 2 P P F 2 plants Phenotypic ratio 3 purple : 1 white Genotypic ratio 1 PP : 2 Pp : 1 pp Sperm p p P PP Pp pp Eggs Punnett square

Homologous chromosomes bear the alleles for each character On homologous chromosomes, alleles of a gene reside at the same locus Homozygous individuals have the same allele on both homologues Heterozygous individuals have a different allele on each homologue Gene loci P a B P a b PP Homozygous for the allele Copyright © 2009 Pearson Education, dominant Inc. Genotype: aa Homozygous for the recessive allele Dominant allele Recessive allele Bb Heterozygous

Mendel’s law of independent assortment Hypothesis: Independent assortment Hypothesis: Dependent assortment P generation rryy RRYY ry Gametes RY F 1 generation rryy RRYY Gametes RY Rr. Yy Sperm 1 – 2 F 2 generation 1 – 2 RY 1 – 2 Eggs 1 – 2 1 – 4 ry RY ry 1 – 4 RY 1 – 4 r. Y Eggs 1 – 4 Hypothesized (not actually seen) ry 1 – 4 Ry ry RY 1 – 4 Sperm 1 – Ry r. Y 4 1 – 4 RRYY Rr. YY RRYy Rr. YY rr. YY Rr. Yy rr. Yy RRYy Rr. Yy RRyy Rr. Yy rr. Yy Rryy rryy Actual results (support hypothesis) Copyright © 2009 Pearson Education, Inc. ry 9 –– 16 3 –– 16 1 –– 16 Yellow round Green round Yellow wrinkled Green wrinkled

Mendel’s law of independent assortment A dihybrid cross Track TWO characters Parental generation: round yellow seeds x wrinkled green seeds F 1 generation: all plants with round yellow seeds F 2 generation: of plants with round yellow seeds of plants with round green seeds of plants with wrinkled yellow seeds of plants with wrinkled green seeds Questions? Why nonparental combinations were observed Why a 9: 3: 3: 1 ratio was observed among the F 2 offspring Copyright © 2009 Pearson Education, Inc.

Mendel’s law of independent assortment Law of independent assortment Each pair of alleles segregates independently of the other pairs of alleles during gamete formation For genotype Rr. Yy, four gamete types are possible: RY, Ry, r. Y, and ry Copyright © 2009 Pearson Education, Inc.

Hypothesis: Independent assortment Hypothesis: Dependent assortment P generation rryy RRYY ry Gametes RY F 1 generation ry Gametes RY Rr. Yy Sperm 1 – 2 F 2 generation rryy RRYY 1 – 2 RY 1 – 2 ry RY Eggs 1 – 2 1 – 4 ry 1 – 4 RY 1 – 4 r. Y Eggs 1 – 4 Hypothesized (not actually seen) 1 – 4 RY 1 – 4 r. Y 1 – 4 Ry 1 – 4 ry RRYY Rr. YY RRYy Rr. YY rr. YY Rr. Yy rr. Yy 9 –– 16 Ry RRYy Rr. Yy RRyy Rr. Yy rr. Yy Rryy ry Actual results (support hypothesis) 3 –– 16 1 –– 16 Yellow round Green round Yellow wrinkled Green wrinkled

Let’s try a dihybrid cross…. Characters: 1. Earlobes- Free is dominant (F) Attached is recessive (f) 2. Tongue rolling- Rollers are dominant (R) Nonrollers are recessive (r) Cross a true breeding unattached roller with an attached nonroller to the F 2 generation Copyright © 2009 Pearson Education, Inc.

Geneticists use the testcross to determine unknown genotypes What if we did NOT know the genotype of our unattached tongue roller? ? Testcross: Mating between an individual of unknown genotype and a homozygous recessive individual Will show whether the unknown genotype includes a recessive allele Used by Mendel to confirm true-breeding genotypes Copyright © 2009 Pearson Education, Inc.

Remember: Mendel’s laws reflect probability and statistics What is the probability that the Olsen sisters have such similar genes but are NOT identical twins? Copyright © 2009 Pearson Education, Inc.

Mendel’s laws reflect the rules of probability The probability of a specific event is the number of ways that event can occur out of the total possible outcomes. 1. Rule of multiplication Multiply the probabilities of events that must occur together Example: Cross two Aa. Bb. Cc parents What is probability that offspring will be aabbcc? 2. Rule of addition Add probabilities of events that can happen in alternate ways Copyright © 2009 Pearson Education, Inc.

F 1 genotypes Bb male Formation of sperm Bb female Formation of eggs 1 – 2 B B b B 1 – 4 1 – 2 b b B 1 – 4 F 2 genotypes b b b 1 – 4

Law of segregation: Allele pairs separate (segregate) from each other during the production of gametes so that a sperm or egg carries only one allele for each gene Law of independent assortment: Each pair of alleles segregates independently of the other pairs of alleles during gamete formation

VARIATIONS ON MENDEL’S LAWS Copyright © 2009 Pearson Education, Inc.

Incomplete dominance results in intermediate phenotypes Incomplete dominance P generation White rr Red RR r Gametes R Neither allele is dominant over the other F 1 generation Pink Rr 1 Heterozygous individual showsan intermediate phenotype Gametes– 2 R 1 – 2 F 2 generation 1 – 2 r Sperm 1 – r R 2 R RR r. R r Rr rr Eggs 1 – 2 Copyright © 2009 Pearson Education, Inc.

Many genes have more than two alleles in the population Multiple alleles A diploid individual can carry any two of these alleles Example: ABO blood group Three alleles: IA, IB, and i Four phenotypes: Type A, Type B, Type AB and Type O blood Copyright © 2009 Pearson Education, Inc.

Many genes have more than two alleles in the population Codominance Neither allele is dominant over the other Expression of both alleles is observed as a distinct phenotype in the heterozygous individual Copyright © 2009 Pearson Education, Inc.

Blood Group (Phenotype) Genotypes Red Blood Cells O ii A IA IA or IA i Carbohydrate A B IB IB or IB i Carbohydrate B AB IA IB

A single gene may affect many phenotypic characters Pleiotropy One gene influencing many characteristics Example: The gene for sickle cell disease Affects the type of hemoglobin produced Affects the shape of red blood cells Causes anemia Causes organ damage Is related to susceptibility to malaria Copyright © 2009 Pearson Education, Inc.

Individual homozygous for sickle-cell allele Sickle-cell (abnormal) hemoglobin Abnormal hemoglobin crystallizes, causing red blood cells to become sickle-shaped Sickle cells Clumping of cells and clogging of small blood vessels Breakdown of red blood cells Physical weakness Impaired mental function Anemia Heart failure Paralysis Pain and fever Pneumonia and other infections Accumulation of sickled cells in spleen Brain damage Damage to other organs Rheumatism Spleen damage Kidney failure

A single character may be influenced by many genes Polygenic inheritance Many genes influence one trait Skin color is affected by at least three genes Copyright © 2009 Pearson Education, Inc. P generation aabbcc (very light) AABBCC (very dark) F 1 generation Aa. Bb. Cc F 2 generation 1 – 8 1 –– 64 6 –– 64 1 – 8 1 Eggs – 8 1 – 8 15 –– 64 Aa. Bb. Cc Sperm 1 – 1 – 8 8 8 20 –– 64 15 –– 64 1 – 8 6 –– 64 1 – 8 1 –– 64

Summary of variations on Mendel’s laws Incomplete dominance White rr Red RR Pink Rr Pleiotropy Single gene Multiple genes Multiple characters Polygenic inheritance Single characters (such as skin color)

INHERITANCE OF GENES

Genetic traits in humans can be tracked through family pedigrees Pedigree: Shows the inheritance of a trait in a family through multiple generations Demonstrates dominant or recessive inheritance Can also be used to deduce genotypes of family members Free earlobe Copyright © 2009 Pearson Education, Inc. Attached earlobe

First generation (grandparents) Ff Second generation (parents, aunts, and uncles) FF or Ff Third generation (two sisters) Female Male Affected Unaffected Ff ff ff ff Ff Ff Ff ff ff FF or Ff

AUTOSOMAL DISORDERS

Types of inherited autosomal diseases 1. Autosomal recessive inheritance Two recessive alleles are needed to show disease Heterozygous parents are carriers Probability of inheritance increases with inbreeding Parents Normal Dd D D Offspring Eggs d Copyright © 2009 Pearson Education, Inc. Normal Dd ´ Sperm d DD Normal Dd Normal (carrier) dd Deaf

Types of inherited autosomal diseases 2. Autosomal dominant inheritance One dominant allele is needed to show disease Dominant lethal alleles are usually eliminated from the population Copyright © 2009 Pearson Education, Inc.

SEX CHROMOSOMES AND SEX-LINKED GENES Copyright © 2009 Pearson Education, Inc.

Chromosomes determine sex in many species (male) 44 + XY X-Y system in mammals, fruit flies XX = female; XY = male X-O system in grasshoppers and roaches XX = female; XO = male Chromosome number in ants and bees Diploid = female; haploid = male Copyright © 2009 Pearson Education, Inc. 22 + X Parents’ diploid cells (female) 44 + XX 22 + Y Sperm 22 + X 44 + XY Offspring (diploid) 22 + XX Egg 22 + X 32 16

Sex-linked genes exhibit a unique pattern of inheritence Sex-linked genes are located on either of the sex chromosomes X-linked genes (Xg) can be passed from: ______ to _______ and ____________ to _______ Y-linked genes (Yg) can be passed from: ______ to _______ Copyright © 2009 Pearson Education, Inc.

Example of sex-linked genes: fruit fly eye color Female Male Xr Y XR XR Sperm Eggs XR Xr Y XR Xr XR Y R = red-eye allele r = white-eye allele

Female Male XR Xr Xr Y Sperm Xr Y XR XR Y Xr Xr Y Eggs

Sex-linked disorders affect mostly males Males express X-linked recessive disorders because they only have ONE copy Hemophilia Colorblindness Queen Victoria Albert Alice Louis Alexandra Czar Nicholas II of Russia Alexis Copyright © 2009 Pearson Education, Inc.

THE CHROMOSOMAL BASIS OF INHERITANCE Copyright © 2009 Pearson Education, Inc.

Chromosome behavior accounts for Mendel’s laws Mendel’s Laws rely on chromosome separation in meiosis The law of segregation depends on separation of homologous chromosomes in anaphase I The law of independent assortment depends on alternative orientations of chromosomes in metaphase I Copyright © 2009 Pearson Education, Inc.

F 1 generation R r All round yellow seeds (Rr. Yy) y Y r R y Y R Y y R R Y y Anaphase I of meiosis r Y R r R Y Metaphase I of meiosis (alternative arrangements) r Metaphase II of meiosis Gametes y r 9 y Y r r r 1 – r. Y 4 : 3 : 1 y Y Y : 3 Y R Fertilization among the F 1 plants F 2 generation R r y 1 – ry 4 1 – RY 4 r y y R R 1 – 4 Ry