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11-3 Exploring Mendelian Genetics11–3 Exploring Mendelian Genetics
11–3 Exploring Mendelian Genetics
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The Law of Segregation
• It states that when an organism produces gametes its two alleles for a trait separate and go into different gametes.
• Therefore, when gametes from 2 parents join to produce a new organism, the offspring receive one allele from each parent.
11–3 Exploring Mendelian Genetics
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Does the gene for one trait have anything to do with whether the gene for another trait is inherited?
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Mendel showed that….Different traits are inherited separately
This is called the Law of Independent Assortment
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11–3 Exploring Mendelian Genetics
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• The alleles for rolling your tongue segregate independently of those for hair or eye color.
• This leads to genetic variation.
Independent Assortment
11–3 Exploring Mendelian Genetics
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Beyond Dominant and Recessive Alleles
Some traits cannot be explained by simple dominant or recessive
alleles.
In fact, most genes have more than 2 alleles.
11–3 Exploring Mendelian Genetics
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Beyond Dominant and Recessive Alleles
Incomplete Dominance
When one allele is not completely dominant over another it is called incomplete dominance.
Blending!
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11–3 Exploring Mendelian Genetics
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A cross between red (RR) and white (WW) snapdragons produces pink-colored flowers (RW).
Beyond Dominant and Recessive Alleles
WW
RR
11–3 Exploring Mendelian Genetics
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Beyond Dominant and Recessive Alleles
Codominance
In codominance, both alleles contribute to the phenotype.
In certain varieties of chicken, the allele for black feathers is codominant with the allele for white feathers.
Heterozygous chickens are speckled with both black and white feathers. The black and white colors do not blend to form a new color, but appear separately and equally.
11–3 Exploring Mendelian Genetics
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11–3 Exploring Mendelian Genetics
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Beyond Dominant and Recessive Alleles
Multiple Alleles
Each organism has only 2 alleles for a trait however, more than 2 alleles exist in a population.
For example: a rabbit’s coat color.
The coat color is determined by a single gene that has at least 4 different alleles.
11–3 Exploring Mendelian Genetics
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Beyond Dominant and Recessive Alleles
Different combinations of alleles result in the colors shown here.
Full color: CC, Ccch, Cch, or CcChinchilla: cchch, cchcch, or cchcHimalayan: chc, or chchFull color: CC, Ccch, Cch, or Cc
KEY
C = full color; dominant to all other alleles
cch = chinchilla; partial defect in pigmentation; dominant to ch and c alleles
ch = Himalayan; color in certain parts of the body; dominant to c allele
c = albino; no color; recessive to all other alleles
11–3 Exploring Mendelian Genetics
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Beyond Dominant and Recessive Alleles
Polygenic Traits
Traits controlled by two or more genes are said to be polygenic traits “having many genes.”
Height in humans is a polygenic trait because it is controlled by multiple genes.
Other examples include: eye color and skin color.
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Pattern of Inheritance Description Example
Incomplete Dominance One allele is not completely dominant over another
Snapdragon plant: A plant with red flowers is crossed with a plant with white flowers, producing heterozygous plants with pink flowers.
Codominance Both alleles contribute to the phenotype
One variety of chicken: A black feathered chicken is crossed with a white feathered chicken, producing heterozygous speckled chickens.
Multiple Alleles A gene has more than two alleles
Rabbit fur color
Polygenic Traits Traits controlled by more than one gene
Human skin color: Several genes control this trait, leading to a wide range of colors.
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