NOTES FOR BIOLOGY 1201

Section 001

Spring 2005

DR. STEVEN POMARICO

>>>>>>Mendelian inheritance has it physical basis in the behavior of chromosomes during sexual life cycles.

                   -Chromosomes and genes are both paired in diploid cells.

                   -Chromosomes separate during the formation of gametes and allele pair                              segregation.

                   -Fertilization restores diploid condition for both chromosomes and genes.

>From these observations the chromosomal theory of inheritance arose.

A genetics and cytology time line

>>>>>>Linked genes tend to be inherited together because they are located on the same

chromosome.

---Linked genes

         -Genes on the same chromosome don’t assort independently.

         -Since independent assortment doesn’t occur, the 9:3:3:1 ratio is not seen

Linking Genes to Chromosomes: The Work of Morgan -9.8.4

>>>>>>Thomas H. Morgan traced a gene to a specific chromosome

Morgan worked with fruit flies, Drosophila melanogaster

Note on genetic symbols:

         -An allele is named after the non-wild-type, or mutant phenotype discovered.

         -The wild-type allele is dominant and the mutant allele is recessive

         -The wild-type trait is designated with a superscript + sign.

                   For eye color the wild-type phenotype is red, the mutant color is white and white is recessive

                             Therefore the symbol for this gene is w

                                       a fly with white eyes = w

                                       A fly with red eyes = w⁺

---Wild-type

---Mutant phenotypes

Morgan’s Conclusions - 9.8.5

>The discovery of a sex-linked gene

Morgan crossed a white-eyed (w) male with a red-eyed (w⁺) female

In the F₁ the white-eyed trait disappeared, therefore the red-eyed trait is dominant.

In the F₂ generation the white-eyed trait reappeared BUT ONLY IN THE MALES!

Morgan deduced that the gene for eye color is located on the sex chromosomes

---Sex-linked genes

Crossing Over and Recombination: A Tool for Mapping Genes - 9.8.2

Morgan found the evidence for linked genes in a dihybrid testcross between flies with autosomal recessive traits and heterozygous wild-type flies.

                             b = black body                vg = vestigial wings

                             b⁺ = gray body                vg⁺ = wild-type wings

The expected 1:1:1:1 phenotype did not occur. Instead more flies had the parental phenotype.

Morgan proposed that this shift toward the parental phenotype was due to linkage of the genes.

>>>>>>Independent assortment of chromosomes and crossing over cause genetic recombination

---Genetic recombination

Remember that recombination of unlinked genes is a result of independent assortment.

Back to Mendel

                   YY or Yy = yellow seeds                 RR or Rr = round seeds

                               yy = green seeds                              rr = wrinkled seeds

Test cross (YyRr x yyrr) => 2 parental types, 2 genetic recombinants

       Yr yr yR yr

_________________________________________

 1/4 YyRr 1/4 yyrr 1/4 yyRr 1/4 Yyrr | yr

_________________ ________________

       1/2 Parental 1/2 Recombinant

-50% recombination expected when 2 genes are located on different chromosomes because of INDEPENDENT ASSORTMENT

---Parental types

---Genetic recombinants

Gene Mapping Using Recombination Frequencies - 9.8.3

>>>Back to the flies and their linked genes.

If the genes were linked, then they should not recombine into assortments not found in parents. Why aren’t ALL of the offspring parental types?

Test cross: b⁺b vg⁺vg x bb vgvg

965 grey/long        944 black/vest. 206 black/long 185 grey/vest.

__________________________ ________________________

                   Parental                                    Recombinants

This cross has a 17% recombination frequency.

                   recombination frequency = # of recombinants / total # of offspring

The recombination was due to crossing over

Sex-Linked Traits in Humans - 9.9.1

>>>>>>The chromosomal basis of sex produces unique patterns of inheritance

Sex is a phenotypic character determined by inherited chromosomes.

>There are different systems for chromosomal sex determination

The X-Y and X-0 systems

         -In these systems the male is heterogametic and the female is                              homogametic

         -The sex of the offspring is determined at fertilization

         -The male to female ratio is 1:1

Heterogametic versus Homogametic

---Heterogametic - two types of gametes

---Homogametic - one type of gamete

=> X-Y: Drosophila, humans, other mammals

         -sex determined by whether male gamete (sperm) bears X or Y

         -for the somatic cells the female is XX and the male is XY

=> X-O: Grasshoppers, roaches, other insects

         -sex determined by number of X chromosomes, 1/2 of the male gametes                    have no sex chromosome

         -for the somatic cells the female is XX and the male is XO

The Z-W systems

         -birds, some fish and insects (butterflies)

         -the female is the heterogametic sex and the male is homogametic

         -for the somatic cells the female is ZW and the male is ZZ

The Haplo-diploid system

         -ants and bees

         -no sex chromosomes present

         -queen lays eggs and only some are fertilized

                   -males hatch from unfertilized eggs (haploid) => parthenogenesis

                   -females from fertilized eggs (diploid)

>>>SEX-LINKED INHERITANCE

         -traits unrelated to sex are on the sex chromosomes

                   -in humans, "sex-linked" usually mean "X chromosome linked"

                   -X chromosome is bigger and therefore more genes possible

                   -most X chromosomes have no homologous loci on the Y chromosome

                             (i.e., can’t be homozygous or heterozygous at these loci)

                                       therefore male is hemizygous

>>>Transmissions of sex-linked genes

                   -Fathers pass X to daughters

                   -Mothers pass X to sons and daughters

                   -recessive sex-linked traits appear in males twice as often as in                              females

Problems in Heredity - 9.10.1

>>>>>>Alterations of chromosome number or structure

Errors during meiosis or the effect of mutagens can result in major chromosomal changes

>>>Changes in chromosomal number: Aneuploidy and polyploidy

Changes in chromosomal number results from a nondisjunction during meiosis (I or II) or mitosis

---Nondisjunction

---Aneuploidy

         -results from the fusion of aberrant gamete with a normal gamete

         -Trisomy => 2N + 1 -- Down's syndrome "Trisomy 21"

         -Monosomy => 2N - 1

         -Mitosis transmits the anomaly

---Polyploidy

         -Triploidy (3N)

         -Tetraploidy (4N)

         -Polyploidy is common in plants