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Chapter 15 Chromosomal Basis of Inheritance. Mendel & Chromosomes Mendel was ahead of his time. 19 th C cytology suggested a mechanism for his earlier.

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Presentation on theme: "Chapter 15 Chromosomal Basis of Inheritance. Mendel & Chromosomes Mendel was ahead of his time. 19 th C cytology suggested a mechanism for his earlier."— Presentation transcript:

1 Chapter 15 Chromosomal Basis of Inheritance

2 Mendel & Chromosomes Mendel was ahead of his time. 19 th C cytology suggested a mechanism for his earlier findings. What did they find?Mendel was ahead of his time. 19 th C cytology suggested a mechanism for his earlier findings. What did they find? Chromosomes and genes are both present in pairs in diploid cells. Chromosomes and genes are both present in pairs in diploid cells. Homologous chromosomes separate and alleles segregate during meiosis. Homologous chromosomes separate and alleles segregate during meiosis. Fertilization restores the paired condition for both chromosomes and genes. Fertilization restores the paired condition for both chromosomes and genes.

3 Chromosome Theory of Inheritance Mendelian genes have specific loci on chromosomesMendelian genes have specific loci on chromosomes Chromosomes are what physically undergo segregation and independent assortment.Chromosomes are what physically undergo segregation and independent assortment.

4 Morgans Fruit Flies Morgan first associated a specific gene with a specific chromosome.Morgan first associated a specific gene with a specific chromosome. Why fruit flies?Why fruit flies? Breed quickly (two week generations)Breed quickly (two week generations) 4 pairs of chromosomes (3 pair of autosomes, 1 pair of sex chromosomes)4 pairs of chromosomes (3 pair of autosomes, 1 pair of sex chromosomes) Females = XXFemales = XX Males = XYMales = XY

5 Morgans Fruit Flies Wild Type flies are the most common natural phenotype. (Red Eyes)Wild Type flies are the most common natural phenotype. (Red Eyes) After a series of crosses, Morgan produced mutants with white eyes.After a series of crosses, Morgan produced mutants with white eyes. After a few generations, Morgan noted that only males displayed the white eyes.After a few generations, Morgan noted that only males displayed the white eyes. He concluded that certain genes are located on the sex chromosome and thus linked to sex.He concluded that certain genes are located on the sex chromosome and thus linked to sex. Sex-linked genes (ie: hemophilia)Sex-linked genes (ie: hemophilia)

6 Sex-linked Traits

7 Morgan concluded the gene with the white-eyed mutation is on the X chromosome. Y chromosome = no infoMorgan concluded the gene with the white-eyed mutation is on the X chromosome. Y chromosome = no info Males (XY) only need one copy of recessive allele to show trait.Males (XY) only need one copy of recessive allele to show trait.

8 Linked Genes All genes located on the same chromosome tend to be inherited together.All genes located on the same chromosome tend to be inherited together. Chromosome passed on as a unit.Chromosome passed on as a unit. Testcross results varied from those predicted by the law of independent assortment.Testcross results varied from those predicted by the law of independent assortment. This showed that certain genes will assort together. (on same chromosome)This showed that certain genes will assort together. (on same chromosome)

9 Linked Genes

10 Body color and wing shape are usually inherited together (same chromosome)Body color and wing shape are usually inherited together (same chromosome)

11 Recombinants Where did the other phenotypes come from? (grey-vestigial and black normal) Where did the other phenotypes come from? (grey-vestigial and black normal) Genetic recombination= offspring with new combinations of traits inherited from two parents Genetic recombination= offspring with new combinations of traits inherited from two parents How?? How?? independent assortment of genes (non- homologous) independent assortment of genes (non- homologous) crossing over of genes (homologous) crossing over of genes (homologous)

12 Recombinants Mendels dihybrid crosses produced recombinant genotypes. Mendels dihybrid crosses produced recombinant genotypes. 50% parental : 50% recombinant genotypes typical for nonhomologues 50% parental : 50% recombinant genotypes typical for nonhomologues Metaphase I Metaphase I YR, Yr, yR, and yr YR, Yr, yR, and yr Seed shape and color tetrads are independent from one another Seed shape and color tetrads are independent from one another

13 Recombinants Linked genes tend to move together during meiosis/fertilization Linked genes tend to move together during meiosis/fertilization If Independent assortment of genes If Independent assortment of genes Expect a 1:1:1:1 phenotype ratio Expect a 1:1:1:1 phenotype ratio If Complete linkage of genes If Complete linkage of genes 1:1:0:0 ratio (all parental) 1:1:0:0 ratio (all parental) Observed 17% recombinant flies Observed 17% recombinant flies Suggested Incomplete linkage of genes Suggested Incomplete linkage of genes

14 Crossing Over Prophase I: homologous chromosomes can swap alleles Prophase I: homologous chromosomes can swap alleles More variable gametes than simple mendelian rules would predict More variable gametes than simple mendelian rules would predict

15 Therefore, Crossing Over Explains:

16 Linkage Maps Ordered list of genetic loci along chromosome Ordered list of genetic loci along chromosome Based on recombination frequencies between two genes Based on recombination frequencies between two genes Higher % of recombination = farther apart Higher % of recombination = farther apart More places in between genes for crossing over to occur and separate the genes More places in between genes for crossing over to occur and separate the genes

17 Linkage Maps The recombination frequency between cn and b is 9%. The recombination frequency between cn and vg is 9.5%. The recombination frequency between b and vg is 17%.

18 Linkage Maps Map units are the distances between genes on a chromosome. Map units are the distances between genes on a chromosome. 1 map unit = 1% recombination 1 map unit = 1% recombination 50% recombination = so far apart that crossing over is all but certain 50% recombination = so far apart that crossing over is all but certain Remember, 50% recomb. = ind. assortment (non-homologous) Remember, 50% recomb. = ind. assortment (non-homologous) Linkage maps show relative order/distance Linkage maps show relative order/distance More recent studies show exact distances and order More recent studies show exact distances and order

19 Sex Chromosomes

20 X-Y Sex Determination X and Y behave as homologues X and Y behave as homologues Each egg receives an X from XX mother Each egg receives an X from XX mother One sperm receives X and one Y One sperm receives X and one Y Results in 50/50 chance of male or female Results in 50/50 chance of male or female SRY Gene SRY Gene Present (on Y) : gonads develop into testes (male) Present (on Y) : gonads develop into testes (male) Not present (no Y): gonads become ovaries (female) Not present (no Y): gonads become ovaries (female) SRY also regulates other genes SRY also regulates other genes

21 Sex-Linked Genes Sex chromosomes also contain other genes. (ie: drosophila eye color) Sex chromosomes also contain other genes. (ie: drosophila eye color) Females must be homozygous recessive to display trait (XX – second X can mask recessive) Females must be homozygous recessive to display trait (XX – second X can mask recessive) Females can be carriers Females can be carriers Males only need to inherit a single copy to show trait Males only need to inherit a single copy to show trait Can a male be a carrier? Can a male be a carrier?

22 Sex-Linked Disorders Duchenne Muscular Dystrophy Duchenne Muscular Dystrophy 1/3500 males 1/3500 males Progressive muscular weakening Progressive muscular weakening Die by mid-20s Die by mid-20s Missing X-linked gene Missing X-linked gene No production of dystrophin (muscle protein) No production of dystrophin (muscle protein)

23 Sex-Linked Disorders Hemophilia Hemophilia Absence of one or more clotting factors Absence of one or more clotting factors affected individuals cannot stop bleeding normally affected individuals cannot stop bleeding normally treated with protein injections treated with protein injections

24 Barr Bodies Only one of the females X chromosomes is active Only one of the females X chromosomes is active The other becomes a Barr body The other becomes a Barr body When assorted into an ovum, the Barr body becomes activated again When assorted into an ovum, the Barr body becomes activated again Which X becomes Barr body is random in each cell Which X becomes Barr body is random in each cell Approx. 50% express each allele (if hetero) Approx. 50% express each allele (if hetero)

25 X-Inactivation in Females

26 Nondisjunction Errors with meiotic spindle Errors with meiotic spindle Meiosis I: Homologous tetrad doesnt separate OR Meiosis I: Homologous tetrad doesnt separate OR Meiosis II: Sister chromatids dont separate Meiosis II: Sister chromatids dont separate Some gametes receive two of the same type of chromosome and another gamete receives no copy Some gametes receive two of the same type of chromosome and another gamete receives no copy

27 Aneuploidy Results from fertilization involving nondisjoined gamete(s) Results from fertilization involving nondisjoined gamete(s) Trisomy three copies of a particular chromosome (2n + 1) Trisomy three copies of a particular chromosome (2n + 1) Monosomy only one copy of a particular chromosome (2n – 1) Monosomy only one copy of a particular chromosome (2n – 1)

28 Down Syndrome Three copies of chromosome 21 Three copies of chromosome 21 1/700 children born each year 1/700 children born each year Definite link with maternal age Definite link with maternal age

29 Aneuploidy in Sex Chromosomes XXY Male (Klinefelters Syndrome) XXY Male (Klinefelters Syndrome) Male sex organs, sterile w/ femininity Male sex organs, sterile w/ femininity XYY Males XYY Males Tend to be taller than normal Tend to be taller than normal

30 Aneuploidy in Sex Chromosomes XXX Females XXX Females Will develop as normal females Will develop as normal females XO Females (monosomy – Turner syndrome) XO Females (monosomy – Turner syndrome) Immature females Immature females 1/2500 live female births 1/2500 live female births

31 Changes in Chromosomes


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