Linkage and gene maps. Sex determination

Июль 27, 2022

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Linkage and gene maps. Sex determination

Содержание

2. QUESTIONS -Morgan”s experiments with fruit flies. -What is Linkage. Types of Likage. -The main statements of
3. Morgan’s Experimental Evidence: Scientific Inquiry The first solid evidence associating a specific gene with a specific
4. Thomas Hunt Morgan in the ‘Fly Lab’
5. Some Mutations
6. Morgan found that body color and wing size are usually inherited together in specific combinations (parental
7. The tendency of different genes present on a chromosome to be inherited together from one generation
8. Types of Linkage: 1. Complete linkage: the characters are inherited together in their original combinations for
9. Test cross: P ♀ aabb x ♂AaBb G ab AB, ab F2 AaBb, aabb Grey, Long
10. However, nonparental phenotypes were also produced Understanding this result involves exploring genetic recombination, the production of
11. Genetic Recombination and Linkage Recombinant offspring are those that show new combinations of the parental traits
12. 2. Incomplete linkage involves separation of linked genes as result of crossing over. They produce some
13. Recombinant Types Produced when a crossover occurs between the 2 genes being studied: A a B
14. Morgan formulated “The chromosome theory of Linkage” according to which: Each chromosome bears many genes. Genes
15. Sturtevant used recombination frequencies to make linkage maps of fruit fly genes Using methods like chromosomal
16. A linkage map is a genetic map of a chromosome based on recombination frequencies Distances between
17. Map Units The farther apart genes are on a chromosome the more likely they are to
18. Sex determination Genetic information on the sex chromosomes is responsible for the primary sex determination. The
19. 1. Number of sex chromosomes. In this method of sex determination, chromosomal number is different in
20. 2. Differences in sex chromosomes XX –XY method In this type of sex determination, both males
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QUESTIONS
-Morgan”s experiments with fruit flies.
-What is Linkage. Types of Likage.
-The main

statements of Chromosomal Theory of Linkage.
-Genetic Maps.
Sex determination and its types
Sex linked inheritance

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Morgan’s Experimental Evidence: Scientific Inquiry
The first solid evidence associating a specific

gene with a specific chromosome came from Thomas Hunt Morgan, an embryologist
Morgan’s experiments with fruit flies provided convincing evidence that chromosomes are the location of Mendel’s heritable factors

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Thomas Hunt Morgan in the ‘Fly Lab’

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Some Mutations

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Morgan found that body color and wing size are usually inherited

together in specific combinations (parental phenotypes)
He noted that these genes do not assort independently, and reasoned that they were on the same chromosome
Each chromosome has hundreds or thousands of genes (except the Y chromosome)
Genes located on the same chromosome that tend to be inherited together are called linked genes

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The tendency of different genes present on a chromosome to be

inherited together from one generation to the next generation is called linkage. These genes do not assort independently.

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Types of Linkage:
1. Complete linkage: the characters are inherited together in

their original combinations for two or more generations in a continues and regular fashion
Parents: P Grey, Long x Black, Vestigial ♀ AA BB ♂ aabb
Gametes: AB ab
F1 generation All Grey, Long
AaBb

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Test cross: P ♀ aabb x ♂AaBb
G ab AB, ab

F2 AaBb, aabb
Grey, Long Black, Vestigial
In this type of linkage, genes are closely associated and tend to transmit together.

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However, nonparental phenotypes were also produced
Understanding this result involves exploring genetic

recombination, the production of offspring with combinations of traits differing from either parent

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Genetic Recombination and Linkage
Recombinant offspring are those that show new combinations

of the parental traits
Morgan discovered that genes can be linked but due to the appearance of recombinant phenotypes, the linkage appeared incomplete
Crossing over of homologous chromosomes was the mechanism

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2. Incomplete linkage involves separation of linked genes as result of

crossing over. They produce some percentages of non-parental combinations.
In this type of linkage, genes are widely located in chromosomes and have chance of separation by crossing over.
Test cross. P Grey, Long (F1) X Black, Vestigial
♀ AaBb x ♂ aabb
G non-crossovers: AB, ab ab
recombinants: Ab, aB
F2 AaBb – grey, long (41,5%)
aabb – black, vestigal (41,5%)
83% - parental combination showing linkage
Aabb – grey, vestigial (8,5%)
aaBb – black, long (8,5%)
17% - recombinants due to crossing over

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Recombinant Types
Produced when a crossover occurs between the 2 genes being

studied:

Recombinant types

Non recombinant types

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Morgan formulated “The chromosome theory of Linkage” according to which:
Each chromosome

bears many genes. Genes are arranged in a linear fashion in the chromosome.
The strength of linkage depends upon the distance between the linked genes in chromosome. The closely located genes show strong linkage while the widely located genes show weak linkage.
Linked genes remain in their original combination during the course of inheritance.
It becomes possible to determine the distances between the genes in a linkage group, their order and may give diagrammatic representation of chromosomes showing the genes as points separated by distances proportional to the amount of crossing-over. Such a diagrammatic representation of the relative distances between linked genes of a chromosome is called genetic map.

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Sturtevant used recombination frequencies to make linkage maps of fruit fly

genes
Using methods like chromosomal banding, geneticists can develop cytogenetic maps of chromosomes
Cytogenetic maps indicate the positions of genes with respect to chromosomal features

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A linkage map is a genetic map of a chromosome based

on recombination frequencies
Distances between genes can be expressed as map units; one map unit, or centimorgan, represents a 1% recombination frequency
Map units indicate relative distance and order, not precise locations of genes

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Map Units
The farther apart genes are on a chromosome the more

likely they are to be separated during crossing over
2 genes on the same chromosome can be located so far apart that the frequency of recombinant types reaches 50%
Same as for genes located on different chromosomes.
These genes will assort independently, even though they are on the same chromosome.

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Sex determination
Genetic information on the sex chromosomes is responsible for

the primary sex determination. The development of gonads in the body is the main character for sex determination.
If sexually reproducing organisms two types of chromosomes are recognized: autosomes and sex chromosomes.
Sex chromosomes are responsible for the sex determination.
If two sex chromosomes are similar (XX) the individual is described as homogametic. It produces similar gametes.
If two sex chromosomes are different (XY) or it contains only one sex chromosomes (XO) the individual is described as heterogametic and it produces two types of gametes.
Sex determination based on:
number of sex chromosomes
differences in sex chromosomes

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1. Number of sex chromosomes.
In this method of sex determination,

chromosomal
number is different in male and female. Ex.: in bugs, grasshoppers and cockroaches females are with two X-chromosomes and males are with one X-chromosome.
So females are homogametic and males are heterogametic. The sex of the offspring depends on the fertilizing sperm:
P ♀ 2A +XX x ♂ 2A +XO
G A + X A + X A + O
F1 2A + XX 2A+ XO

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2. Differences in sex chromosomes
XX –XY method
In this type

of sex determination, both males and females have the same number of chromosomes:
Females – XX – chromosomes, males-XY chromosomes
P ♀ 2A +XX x ♂ 2A +XY
G A +X A +X A +Y
F1 2A + AA 2A + XY
Ex.: in human beings and Drosophila females are homogametic and males are heterogametic
ZW – ZZ method
In birds, reptiles, some fishes, butterflies, female are heterogametic with ZW-chromosomes and males are homogametic with ZZ-chromosomes