patterns of inheritance chapter 9. gregor mendel deduced the fundamental principles of genetics...
TRANSCRIPT
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Patterns of Inheritance
Chapter 9
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Gregor Mendel
• Deduced the fundamental principles of genetics
• Cross-fertilization
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What’s with the Peas?
• Little spontaneous variation between generations
• Can self-fertilize• Easy to control pollination• Possessed several easily
observable traits– Pea form– Pea color– Flower location– Flower color– Stem size
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What do we get??
• Genetic cross– Bb x Bb
• P generation
• F1 generation
• F2 generation
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Monohybrid Cross• Cross between parent plants that
differ in only one characteristic– Mendel developed four hypotheses
from the monohybrid cross:• There are alternative forms of genes
– Alleles
• For each characteristic, an organism inherits two alleles
– One from each parent
• Alleles can be dominant or recessive• Gametes carry only one allele for each
inherited characteristic
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Mendel’s Laws
• Genes
– Set of instructions that
determine characteristics of an
organism
– Segments of nucleic acid that
specifies a trait
– Found at designated place on
chromosomes
• Locus
– Not all copies of a gene are
identical
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Mendel’s Laws
• Alternative forms of a gene lead to
the alternative form of a trait
– Alleles
• way of identifying the two members
of a gene pair which produce
opposite contrasting phenotypes
• Chromosomes that are homologous
are members of a pair and carry
genes for the same traits in the same
order
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Genes v. alleles
• Genes– Basic instruction
– Sequence of DNA
– General
– Hair color
• Alleles– Variations of that
instruction
– Specifics
– Brown hair
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Genotype verse Phenotype
• Genotype – the alleles an individual receives at fertilization
• Homozygous – an organism has two identical alleles at a gene locus
• Heterozygous – an organism has two different alleles at a gene locus
• Phenotype – the physical appearance of the individual
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Describing GenotypesHomozygous Dominant
when both alleles are dominantBB
Homozygous Recessivewhen both alleles are recessivebb
Heterozygouswhen one allele is dominant and one is recessiveBb
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Punnet Square…..
Genetic cross determines arrangement
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Pedigree Chart
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Phenotypes are not always a direct translation of genotype
Phenotypes may also be influenced by the environment
Examples?• skin color influenced by sun• height/weight influenced by nutrition• animal coat influenced by climate
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Remember…..
P = G + EP = G + E
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Types of Phenotypic Traits
• Discrete traits
• Quantitative traits
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1) Discrete Traits: determined by the action of a single gene
Only a few distinct categories exist for trait
Types of Phenotypic Traits
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Shape of human hairline is a discrete trait
There are 2 alleles and 2 varieties:
Widow’s Peak = dominant
No Widow’s Peak = recessive
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Human earlobe type is a discrete trait
There are 2 alleles and 2 varieties:
Attached earlobe = recessive
Free earlobe = dominant
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Types of Phenotypic Traits
2) Quantitative Traits: determined by 2 or more genes
Has a range of phenotypes for that trait
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Height is a quantitative traitThere’s a range of possible values
Others: weight, skin color
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Past the Peas:Types of Dominance
1) Complete Dominance: 1 or other
2) Incomplete Dominance: 3rd effect
3) Codominance: some of both
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Complete Dominance
Dominant Allele is always expressed when present
Recessive Allele is only expressed as homozygote
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Incomplete Dominance
Alleles have combined (equal) effect on phenotype of heterozygote
Phenotype is intermediate
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Codominance
Both alleles are visible in the phenotype of the heterozygote
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Multiple Allelism: existence of more than 2 alleles of gene
Example: Blood type (A, B, O)
RememberRemember: Each person still only has 2 alleles for that trait, but more than 2 exist
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Multiple Allelism: Blood typingABO Blood Type in Humans exhibits multiple allelism
Phenotype Genotype
O OO
A AA or AO
B BB or BO
AB AB *
How many ALLELES are there?
How many Phenotypes are there?
How many Genotypes are there?
3 ( A, B, O)
4 (A, B, AB, O)
6
What is the relationship between A and B alleles? codominance
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Question: If a woman with blood type O marries a man with blood type B, can they have a child with
blood type A?Phenotype Genotype
O OO
A AA or AO
B BB or BO
AB AB*
No. The mother’s genotype must be OO and the father’s either BB or BO. Their child will either be type B (BO) or type O (OO)
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Sex-linked Traits
Female XX Male XY
Genes located on the X or Y chromosome are sex-linked
X and Y chromosomes are not homologous, they contain different genes
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Sex-linked traits
• Sex chromosomes– Are designated X and Y
– Determine an individual’s sex
– Influence the inheritance of certain traits
• Sex-linked genes– Are any genes located on a sex
chromosome
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Sex-Linked Traits
Females (XX) have 2 copies of each gene on the X chromosome
Males (XY) have only 1 copy of each gene on the X chromosome
Males ALWAYS show X-linked alleles, regardless of dominance
Females can show a dominant condition if present on 1 or both X chromosomes
Females can only show a recessive condition if present on both X chromosomes
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Sex-Linked Disorders in Humans
• number of human conditions result from sex-linked (X-linked) genes
• Red-green color blindness– characterized by a
malfunction of light-sensitive cells in the eyes
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Question…..
• Will a mother that is colorblind automatically have a son that is colorblind?
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Question…..
• Will a mother that is colorblind automatically have a son that is colorblind?
Yes!!!!!
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Beyond Simple Inheritance Patterns
• Polygenic Inheritance – Occurs when a trait is
governed by two or more sets of alleles
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Mutations
• Changes to the nucleotide sequence of the genetic material of an organism
• Can be caused by:– copying errors in the genetic
material during cell division– exposure to UV light or chemical
mutagens– Viruses– can be induced by the organism
itself
• Create variety within gene pool• Less favorable verse more favorable
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Recessive Disorders
• Most human genetic disorders are recessive– Individuals can be
carriers of these diseases
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Dominant Disorders
• Some human genetic disorders are dominant– Achondroplasia is a
form of dwarfism
– Huntington's disease