Genetics and Inheritance

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  • Created by: Ikra Amin
  • Created on: 25-11-14 14:24

Introduction

  • A gene is a sequence of bases on DNA that codes for one polypeptide (protein) 
  • Alleles are alternate versions of the same gene, which are positioned at the same place (locus), on homologous chromosomes. Although many genes have more than one allelic form, any individual cell will only have 2 alleles for each gene, one on each of the homologous chromosomes that carry these alleles. They may be identical allels, (homozygous) or different, (heterozygous).
  • The genotype is the genetic consititution of an organism and is determined by the combination of alleles that an organism inherits from its parents. 
  • The expression of the genotype and its interaction with the environment is known as the phenotype. (All those used in A-level genetics cross q's have NO ENVIRONMENTAL INPUT)
  • Genetic problems try to predict which phenotypes will appear in offspring by carrying out planning breeding experiments (use pure breeding plants) - pure breeding plants: male & female pure breeding parents are chosen i.e. when self fertilised, always produce identical offspring, generation after generation. (Homologous). If inheritance of one pair of alleles is studied it is called a monohybrid cross. 
  • Sometimes you're asked to work out what genotypes exist in whole families, based on knowledge of patterns of inheritance in a family tree. This is a pedigree analysis and is a common way of investigating human genetics, where planned breeding isn't possible.
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Introduction

In order to investigate inheritance of specific genes/alleles it is necessary to choose a suitable organism based on several factors. These features are important because: 

  • The organism must show some discontinuous variation: so you can clearly distinguish the characteristics tested - no intermediates 
  • It must reproduce sexually: so you can see the effect of recombining alleles 
  • It is pure breeding and its mating can be controlled: so alleles of the parent are known
  • It has a short life cycle and produces large numbers of offspring: lots of data in a short time and enough for a stats analysis
  • It is convenient to handle: small, cheap to keep/feed
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Mendelian Genetics

Mendel studied inheritance using pure-breeding individual pea plants. He looked at many characteristics in the pea plants and his findings are the basis of modern genetics. These peas had the advantage of showing many clearly differentiated traits: 

Stems: Tall (dominant trait) / Short (recessive trait) / Flowers on the side (dominant trait) / Flowers on the end (recessive trait)

Pods: Green (dominant trait) / yellow (recessive trait) / inflated (dominant trait) / pinched (recessive trait)

Seeds/flowers: Round (dominant trait) / wrinkled (recessive trait) / yellow (dominant trait) / green (recessive trait) / red flowers (dominant trait) / white (recessive trait)


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1) Monohybrid cross

1. Mendel crossed a pure-breeding tall plant with a pure-breeding dwarf plant and produced all tall offspring. This is called the F1 generation. 

Parental phenotypes:  T   x   t            T=Female (tall) t=male (dwarf)

F1 generation: All Tt (ALL WERE TALL) 

F1 phenotype: tall 

F1 genotype: heterozygous 

Gametes = contain 1 allele from each pair of homologous chromosomes

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2) Monohybrid cross

2. When this F1 generation was crossed amongst themselves, he always got 3/4 tall plants and 1/4 dwarf plants in the next generation (F2 gen) - a ratio of 3:1. This is called the phenotypic ratio.

From these results, Mendel concluded that characteristics were passed from one generation to another via gametes, and that parents must posess two units of information for each characteristic. 

These 'units' we now know as alleles and the conclusion Mendel came to is known as Mendel's first law, the Law of Segregation: The characteristics of an organism are determined by alleles which occur in pairs. Only one of the pair of alleles can be represented in a single gamete. 

Modern interpretation 

  • 1. The gene for height has 2 allelic forms, tall and dwarf. The tall allele being dominant to the dward allele, which is thus termed recessive.
  • 2. Each gamete contains one allele for height. Each individual plant has 2 alleles for height in each cell. 
  • 3. Dominant alleles are=capital letters, recessive alleles=lower case.
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Crossing the parents

Parental phenotypes:       T (tall)            t (dwarf)

Parental genotypes:        TT                  tt 

Gametes:                      All T               All tt 

F1 genotypes:               All Tt 

F1 phenotypes:             All tall 

Crossing the F1 generation:

F1 phenotypes:           All tall (T) 

F1 genotypes:            Tt   Tt   Tt   Tt 

Gametes:                  T,   t,    T,    t

F2 genotypes:           Tt,   TT,   tT,   tt (1st 3 = tall) (last one=dwarf). 

3:1 ratio (3 = tall, 1 = dwarf)

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Crossing the parents cont...

The ratios represent the probability of obtaining each phenotype. Thus in the F2 generation there is a 25% /  0.25 /  1 in 4 probability in obtaining a dwarf plant. This probability does not change as a result of previous offspring as there is random assortment and fertilisation of the same types of alleles each time. 

2) Punnett squares

To avoid confusion when drawing lines to show crosses, a punnett square can be used, where male gametes appear on the TOP and female gametes down the side of the squares. For example: 

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Homozygous and heterozygous

You can see then there are two different genotypes for plants with the phenotype tall. They can be TT / Tt 

The TT genotype is called homozygous or pure breeding

The Tt genotype is called heterozygous or hybrid 

In summary there are 3 different genotypes but only 2 different phenotypes:

Genotype: TT  Phenotype = Tall 

Genotype: Tt  Phenotype = Tall 

Genotype: tt  Phenotype = Dwarf/Small 

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3) Testcross/backcross

This is a way of determining the genotype of an unknown organism. To do this you cross the unknown with a RECESSIVE PHENOTYPE, as these always have a HOMOZYGOUS genotype. 

If any resulting offspring are the homozygous recessive phenotype, the unknown genoype must be HETEROZYGOUS/ Tt 

If ALL offspring have the dominant phenotype, the UNKNOWN genotype must  be HOMOZYGOUS DOMINANT/ TT 

4) Codominance/incomplete dominance

Sometimes a gene has 2 alleles, neither of which is able to dominate, resulting in BOTH ALLELES BEING EXPRESSED IN THE PHENOTYPE. This means that a third phenotype arises when there is a HETEROZYGOUS genotype. E.g. coat colour in cats.  (look in pack for example-pg 8)

5) Multiple alleles and blood groups

Some genes have more than 2 allelic forms, e.g. flower colour in some plants may have whiter, red etc forms. Each individual can only have 2 alleles for each gene, but the combinations are more varied so the patterns of dominance need to be studied. The ABO blood grouping system is determined by 1 gene, with 3 allelic forms. These give rise to 4 blood groups (phenotypes). 

Alleles Ia and Ib are co dominant. Allele Io is recessive to both alleles Ia and Ib.

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6)sex determination

Gender is determined by the combination of chromosomes in a zygote, rather than the genes themselves. The sex chromosomes are of 1 of 2 types, which may be large, X chromosomes, or small, Y chromosomes. 

Autosomes=22 chromosomes. Last 2 chromosomes=pair of sex chromosomes. 

A) Human females have 2 X chromosomes. Their genotype is sometimes describes as 22XX, meaning 22 pairs of homologous chromosomes, plus 2X more. 

B) Human males have an X and a Y chromosome and are described as 22XY. The ova of all females have 22 single chromosomes plus 1 X. The sperm will contain 22 single chromosomes and either an X or a Y chromosome. Thus, it is the male gamete that determines the sex of the offspring. 

C) There is always a 50% probability of 2 parents producing a male child and the same probability of a female child. 

Y shorter so has fewer genes.

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7) Sex linkage

Many genes carried on sex chromosomes are unrelated to sexual characteristics but are inherited in different ways in males and females. These are said to be sex-linked genes and characteristics. 

In males, the sex chromosomes are non-homologous. 

Males are more likely to have a diseases caused by allele as only need 1 but females need both. 

Alleles on X chromosome but no corresponding on Y

Allele on X chromosome and corresponding allele on Y

Any genes on the non-homologous section of the X chromosome will not have an equivalent allele on the Y chromosome. This means that a recessive allele on an X chromosome would have much more chance of being expressed in the phenotype of a male. When answering q's on sex linked characteristics, you need to take into account, and identify, both the sex and the phenotype for the condition. 

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in looking for evidence of sex linkage

1) Look for much greater numbers of affected males than females, or no affected females at all (there's got to be a lot of offspring to be confident)

2) Affected mother with a normal son - this CANNOT happen if the gene is a sex-linked RECESSIVE one (son would also be affected because he would inherit X with recessive allele from mother and Y from father) 

3) Affected daughter with normal father- this CANNOT happen if the gene is sex-linked and RECESSIVE (daughter would be normal as she would inherit X with dominant allele from her father)

Some sex-linked diseases are caused by a DOMINANT allele. 

Look in pack for chi-square notes

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