Chapter 4
Download
1 / 34

Fig. 4-1 - PowerPoint PPT Presentation


  • 85 Views
  • Uploaded on

Chapter 4 overview. Fig. 4-1. Genetic recombination : mixing of genes during gametogenesis that produces gametes with combinations of genes that are different from the combinations received from parents. Independent assortment of homologous chromosomes (Anaphase I). Genes on non-

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about ' Fig. 4-1' - vera-giles


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

Chapter 4

overview

Fig. 4-1


  • Genetic recombination: mixing of genes during

  • gametogenesis that produces gametes with

  • combinations of genes that are different from

  • the combinations received from parents.

  • Independent assortment of homologous

  • chromosomes (Anaphase I). Genes on non-

  • homologous chromosomes (unlinked genes)

  • assort independently.



Using a testcross

to distinguish gamete genotypes

Fig. 4-7


50% = independent

assortment

(genes are not linked)

Fig. 4-8


  • Genetic recombination: mixing of genes during

  • gametogenesis that produces gametes with

  • combinations of genes that are different from

  • the combinations received from parents.

  • Independent assortment of homologous

  • chromosomes (Anaphase I). Genes on non-

  • homologous chromosomes (unlinked genes)

  • assort independently.

  • Crossing over (recombination among linked

  • genes)


cis linked: both dominant alleles on the same homolog

trans linked: dominant alleles on different homologs

Fig. 4-2



  • Crossing over

  • Physical exchanges among non-sister chromatids;

  • visualized cytologically as chiasmata

  • Typically, several crossing over events occur within

  • each tetrad in each meiosis (chiasmata physically

  • hold homologous chromosome together and assure

  • proper segregation at Anaphase I)

p. 115


Crossing over occurs at the four-strand stage

(pre-meiotic G2 or very early prophase I)

Fig. 4-4



  • Crossing over meiosis

  • Physical exchanges among non-sister chromatids;

  • visualized cytologically as chiasmata

  • Typically, several crossing over events occur within

  • each tetrad in each meiosis (chiasmata physically

  • hold homologous chromosome together and assure

  • proper segregation at Anaphase I)

  • The sites at which crossing over occur are random

  • The likelihood that a crossover occurs between any

  • two particular sites (genes) is a function of the

  • physical distance between those two sites


Crossing over usually affects a minority of chromatids in a collection of meioses – recombinants are typically a minority of products

Fig. 4-9


<50% = linked genes collection of meioses – recombinants are typically a minority of products

Fig. 4-10


A.H. Sturtevant (1911-3): frequency of crossing over collection of meioses – recombinants are typically a minority of products

between two genes is a function of their distance

apart on the chromosome; created the first genetic map

number of recombinants

Recombination frequency =

total number of progeny

One map unit = one centimorgan = 1% recombinants


  • Rationales: collection of meioses – recombinants are typically a minority of products

    • Crossover events are random

    • Greater separation, greater likelihood that crossover will occur

    • Map distance should be sum of smaller intervals

    • Construct entire chromosome maps by mapping intervals

    • Linear map correlates with linear chromosome

Fig. 4-11


Markers used in trihybrid testcross collection of meioses – recombinants are typically a minority of products

in Drosophila

v = vermilion eyes (red eyes; v+are red-brown)

cv = crossveinless (cv+ wings have crossveins)

ct = cut wing (ct+ wings have regular margins)


Data from three-point testcross collection of meioses – recombinants are typically a minority of products

v+/ v cv+/ cv ct+/ ct X v / v cv / cv ct / ct

(trihybrid) (tester)

Progeny phenotypes

v cv+ ct+ 580

v+ cv ct 592

v cv ct+ 45

v+ cv+ ct 40

v cv ct 89

v+ cv+ ct+ 94

v cv+ ct 3

v+ cv ct+ 5

1448



Data from three-point testcross collection of meioses – recombinants are typically a minority of products

v+/ v cv+/ cv ct+/ ct X v / v cv / cv ct / ct

(trihybrid) (tester)

Progeny phenotypes

v cv+ ct+ 580

v+ cv ct 592

v cv ct+ 45

v+ cv+ ct 40

v cv ct 89

v+ cv+ ct+ 94

v cv+ ct 3

v+ cv ct+ 5

1448


  • Steps in solving three-point testcross problem collection of meioses – recombinants are typically a minority of products

  • Anticipate and identify eight types of products (23)

  • Identify pairs of reciprocal products

  • Identify parental types as the most frequent pair of

  • products

  • Identify double crossover products as least frequent

  • pair of products


Data from three-point testcross collection of meioses – recombinants are typically a minority of products

v+/ v cv+/ cv ct+/ ct X v / v cv / cv ct / ct

(trihybrid) (tester)

Progeny phenotypes

v cv+ ct+ 580

v+ cv ct 592

v cv ct+ 45

v+ cv+ ct 40

v cv ct 89

v+ cv+ ct+ 94

v cv+ ct 3

v+ cv ct+ 5

1448

Parental types - nco

sco

sco

dco


  • Steps in solving three-point testcross problem collection of meioses – recombinants are typically a minority of products

  • Anticipate and identify eight types of products (23)

  • Identify pairs of reciprocal products

  • Identify parental types as the most frequent pair of

  • products

  • Identify double crossover products as least frequent

  • pair of products

  • Compare the parental and double crossover products

  • to deduce the order of the three gene loci


Fig. 4-12 collection of meioses – recombinants are typically a minority of products

In dco products, the central marker is displaced

relative to the parental types


Fig. 4-13 collection of meioses – recombinants are typically a minority of products


  • Steps in solving three-point testcross problem collection of meioses – recombinants are typically a minority of products

  • Anticipate and identify eight types of products (23)

  • Identify pairs of reciprocal products

  • Identify parental types as the most frequent pair of

  • products

  • Identify double crossover products as least frequent

  • pair of products

  • Compare the parental and double crossover products

  • to deduce the order of the three gene loci

  • Compute map distances by breaking down the

  • results for each interval


Fig. 4-12 collection of meioses – recombinants are typically a minority of products

85 + 8

1448

(0.064)

183 + 8

1448

(0.132)

RF =


Fig. 4-12 collection of meioses – recombinants are typically a minority of products

85 + 8

1448

(0.064)

183 + 8

1448

(0.132)

RF =

13.2 m.u. 6.4 m.u.

v ct cv


Interference collection of meioses – recombinants are typically a minority of products: crossing over in one region

interferes with simultaneous crossing over in

adjacent regions

Expected frequency of dco = product of frequency crossovers in two regions

0.132 X 0.064 = 0.0084

0.084 X 1448 = 12 expected (if two sco are independent events)


Interference collection of meioses – recombinants are typically a minority of products: crossing over in one region

interferes with simultaneous crossing over in

adjacent regions

Expected frequency of dco = product of frequency crossovers in two regions

0.132 X 0.064 = 0.0084

0.084 X 1448 = 12 expected (if two sco are independent events)

Coefficient of coincidence = observed dco / expected dco

8 / 12 = 0.667

Interference = 1 – coefficient of coincidence

1 – 0.667 = 0.333


Fig. 4-14 collection of meioses – recombinants are typically a minority of products

Tomato karyotype (n=12)


Tomato linkage map collection of meioses – recombinants are typically a minority of products

(1952)

Fig. 4-14


Typical phenotypic ratios for a variety of crosses collection of meioses – recombinants are typically a minority of products

(complete allele dominance)

p. 136


ad