4.3 Theoretical genetics & 10.2 HL Genetics

4.3 Theoretical genetics: Objectives 1- Define genotype, phenotype, dominant allele, recessive allele, codominant alleles, locus, homozygous, heterozygous, carrier and test cross. 2- Determine the genotypes and phenotypes of the offspring of a monohybrid cross using a Punnett grid. 3- State that some genes have more than two alleles (multiple alleles). 4- Describe ABO blood groups as an example of codominance and multiple alleles. 5- Explain how the sex chromosomes control gender by referring to the inheritance of X and Y chromosomes in humans.

6- State that some genes are present on the X chromosome and absent from the shorter Y chromosome in humans. 7- Define sex linkage. 8- Describe the inheritance of color blindness and hemophilia as examples of sex linkage. 9- State that a human female can be homozygous or heterozygous with respect to sex-linked genes. 10- Explain that female carriers are heterozygous for X-linked recessive alleles. 11- Predict the genotypic and phenotypic ratios of offspring of monohybrid crosses involving any of the above patterns of inheritance. 12- Deduce the genotypes and phenotypes of individuals in pedigree charts. Heredity SL.pptx Heredity Student version.pptx heredity.ppt

Definitions Allele: an alternative form of a gene, occupying a specific locus. Genotype: the genetic constitution of an organism Phenotype: the characteristics or appearance (structural, biochemical..) of an organism Dominant allele: is the allele that has the same effect on the phenotype whether it is present in the homozygous or heterozygous state. recessive allele: an allele that has an effect on the phenotype only when present in the homozygous state. codominant alleles: Pairs of allele that both affect phenotype when present in a heterozygous state. locus: particular position on the homologous chromosomes of a gene Homozygous: having two identical alleles of a gene heterozygous: having two different alleles of a gene. carrier: an individual that has one copy of a recessive allele that causes a genetic diseases in individuals that are homozygous. test cross: testing a suspected heterozygote by crossing it with a known homozygous recessive.

Flower color White Axial Purple Flower positionTerminal Yellow Seed color Green Round Seed shapeWrinkled Inflated Pod shape Constricted Green Pod colorYellow Tall Stem lengthDwarf

Mendelain Genetics  Example of a monohybrid cross  Mendel needed to explain Copyright © 2009 Pearson Education, Inc.

P generation (true-breeding parents) Purple flowers White flowers F 1 generation All plants have purple flowers F 2 generation Fertilization among F 1 plants (F 1  F 1 ) of plants have purple flowers 3–43–4 of plants have white flowers 1–41–4

Mendel’s law of segregation describes the inheritance of a single character  Four Hypotheses Copyright © 2009 Pearson Education, Inc.

Mendel’s law of segregation describes the inheritance of a single character  Four Hypotheses Law of segregation: Copyright © 2009 Pearson Education, Inc.

P plants 1–21–2 1–21–2 Genotypic ratio 1 PP : 2 Pp : 1 pp Phenotypic ratio 3 purple : 1 white F 1 plants (hybrids) Gametes Genetic makeup (alleles) All All Pp Sperm Eggs PP p ppPp P p P p P P p PP pp All Gametes F 2 plants

Homologous chromosomes show the alleles for each character  For a pair of homologous chromosomes, alleles of a gene locate at the same locus - Copyright © 2009 Pearson Education, Inc.

Gene loci Homozygous for the dominant allele Dominant allele Homozygous for the recessive allele Heterozygous Recessive allele Genotype: P B a P PP a aa b Bb

Geneticists use the testcross to determine unknown genotypes  Testcross Copyright © 2009 Pearson Education, Inc.

B_ or Two possibilities for the black dog: Testcross: Genotypes Gametes Offspring1 black : 1 chocolate All black Bb bb BB Bbbb B b Bb b b B

Genetic traits in humans can be tracked through family pedigrees  A pedigree Copyright © 2009 Pearson Education, Inc.

Freckles Widow’s peak Free earlobe No freckles Straight hairline Attached earlobe Dominant Traits Recessive Traits

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

Many inherited disorders in humans are controlled by a single gene  Inherited human disorders show Recessive inheritance A- B- C- Dominant inheritance A- B- Copyright © 2009 Pearson Education, Inc.

Parents Normal Dd Offspring Sperm Eggs dd Deaf d Dd Normal (carrier) DD Normal D D d Dd Normal (carrier) Normal Dd 

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Incomplete (codominance) dominance results in intermediate phenotypes  Incomplete (codominance) dominance Copyright © 2009 Pearson Education, Inc.

P generation 1–21–2 1–21–2 1–21–2 1–21–2 1–21–2 1–21–2 F 1 generation F 2 generation Red RR Gametes Eggs Sperm RR rR Rrrr R r R r R r Pink Rr R r White rr

Many genes have more than two alleles in the population  Multiple alleles A- B- C- Copyright © 2009 Pearson Education, Inc.

Many genes have more than two alleles in the population  Codominance A- B- C- Copyright © 2009 Pearson Education, Inc.

Blood Group (Phenotype) Genotypes O A ii I A or I A i Red Blood Cells Carbohydrate A Antibodies Present in Blood Anti-A Anti-B Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left Anti-B O AB AB B I B or I B i Carbohydrate B AB IAIBIAIB — Anti-A

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

Antibodies Present in Blood Anti-A Anti-B Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left Anti-B O A B AB — Anti-A Blood Group (Phenotype) O A B AB

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

Female Male X R X r Y X R Y X R X r Y XrXr XRXR Sperm Eggs R = red-eye allele r = white-eye allele

Female Male X R X r X R Y X R Y XRXR XRXR Sperm Eggs X r X R X r Y XrXr

Female Male X R X r X r Y X R Y X R Y XrXr XRXR Sperm Eggs X r X r Y XrXr

 Sex-linked genes are located on either of the sex chromosomes A- B- C- Sex-linked genes exhibit a unique pattern of inheritance Copyright © 2009 Pearson Education, Inc.

HUMAN SEX LINKED TRAITS Copyright © 2009 Pearson Education, Inc. In humans, as in drosophilas, the Y chromosome carries much less genetic information than the X chromosome. 1)One of the X linked inheritance is color blindness called Daltonism. They cannot distinguish red&green colors X R X R  X R X r  X r X r  X R Y  X r Y 

Example Predict the ratio of offspring phenotypes for the following: Female heterozygous for colorblindness  colorblind male?

Example 2) Haemophilia is a disease in which blood clotting doesn’t occur. Clotting is a complex series of reactions. Each step of it requires specific enzymes. One of them is controlled by a gene on the X chromosome called factor VIII(antihaemophilic globin) X H X H  X H X h  X h X h  X H Y  X h Y 

Queen Victoria Albert Alice Louis Alexandra Czar Nicholas II of Russia Alexis There is no prior history of hemophilia in Queen Victoria’s family, so it is thought that the allele arose by spontaneous mutation in the sex cells of one of her parents

HL TOPICS 10.2 Dihybrid crosses and gene linkage Calculate and predict the genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes. Distinguish between autosomes and sex chromosomes. Explain how crossing over between non-sister chromatids of a homologous pair in prophase I can result in an exchange of alleles. Define linkage Explain an example of a cross between two linked genes. Identify which of the offspring are recombinants in a dihybrid cross involving linked genes.

The law of independent assortment is revealed by tracking two characters at once  Example of a dihybrid cross Parental generation: round yellow seeds  wrinkled green seeds  Mendel needed to explain Copyright © 2009 Pearson Education, Inc.

The law of independent assortment is revealed by tracking two characters at once  Law of independent assortment Copyright © 2009 Pearson Education, Inc.

P generation 1–21–2 Hypothesis: Dependent assortment Hypothesis: Independent assortment 1–21–2 1–21–2 1–21–2 1–41–4 1–41–4 1–41–4 1–41–4 1–41–4 1–41–4 1–41–4 1–41–4 9 –– 16 3 –– 16 3 –– 16 1 –– 16 RRYY Gametes Eggs F 1 generation Sperm F 2 generation Eggs Gametes rryy RrYy ry RY ry RY ry RY Hypothesized (not actually seen) Actual results (support hypothesis) RRYY rryy RrYy ry RY RRYY rryy RrYy ry RY RrYy rrYYRrYY RRYyRrYY RRYy rrYy Rryy RRyy rY Ry ry Yellow round Green round Green wrinkled Yellow wrinkled RY rY Ry

Genes on the same chromosome tend to be inherited together  Linked Genes A- B- Example studied by Bateson and Punnett A- B- C- Copyright © 2009 Pearson Education, Inc.

Purple long Purple round Red long Red round Explanation: linked genes Parental diploid cell PpLl Experiment Purple flower PpLl Long pollen PpLl Prediction (9:3:3:1) Observed offspring Phenotypes Most gametes Meiosis PL pl PL pl Fertilization Sperm Most offspring Eggs 3 purple long : 1 red round Not accounted for: purple round and red long PL pl PL pl

Crossing over produces new combinations of alleles  Linked alleles can be separated by crossing over A- B- C- Or Copyright © 2009 Pearson Education, Inc.

Gametes Tetrad Crossing over Baba a b A B A B A b

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

Mutant phenotypes Short aristae Black body (g) Cinnabar eyes (c) Vestigial wings (l) Brown eyes Long aristae (appendages on head) Gray body (G) Red eyes (C) Normal wings (L) Red eyes Wild-type phenotypes

A single character may be influenced by many genes  Polygenic inheritance A- B- Copyright © 2009 Pearson Education, Inc.

P generation 1–81–8 F 1 generation F 2 generation Fraction of population Skin color Eggs Sperm 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 aabbcc (very light) AABBCC (very dark) AaBbCc 1 –– –– 64 6 –– 64 1 –– –– 64 6 –– –– 64 1 –– –– 64 6 –– –– 64

P generation 1–81–8 F 1 generation F 2 generation Eggs Sperm 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 aabbcc (very light) AABBCC (very dark) AaBbCc 1 –– –– 64 6 –– 64 1 –– –– 64 6 –– –– 64

Fraction of population Skin color 1 –– –– 64 6 –– –– 64