Inheritance Patterns
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Inheritance Patterns
Gregor Mendel
Documented a mechanism of inheritance through his experiments with garden peas
Mendel discovered
A ratio of about three to one, purple to white flowers, in the F2 generation
EXPERIMENT True-breeding purple-flowered pea plants and white-flowered pea plants were crossed (symbolized by ×). The resulting F1 hybrids were allowed to self-pollinate or were crosspollinated with other F1 hybrids. Flower color was then observed in the F2 generation.
RESULTS Both purple-flowered plants and whiteflowered plants appeared in the F2 generation. In Mendel’s experiment, 705 plants had purple flowers, and 224 had white flowers, a ratio of about 3 purple : 1 white.
Practice Punnett Squares Ratio – Proportion –
Ratio –
18.2/18.4
Proportion –
Mendel observed the same pattern
In many other pea plant characters
Mendel’s Hypothesis
First, alternative versions of genes
Account for variationsAlleleinfor purple inherited flowers characters, which are now called alleles Locus for flower-color gene
18.1 Allele for white flowers
Homologous pair of chromosomes
Mendel’s Hypothesis
Second, for each character
18.1
An organism inherits two alleles, one from each parent A genetic locus is actually represented twice
Mendel’s Hypothesis
Third, if the two alleles at a locus differ
18.1/ 18.5
Then one, the dominant allele, determines the organism’s appearance The other allele, the recessive allele, has no noticeable effect on the organism’s appearance
Mendel’s Hypothesis
Fourth, the law of segregation
18.1
The two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes
More Genetic Vocabulary
An organism that is homozygous for a particular gene
An organism that is heterozygous for a particular gene
18.5
Has a pair of identical alleles for that gene True-breeding
Has a pair of alleles that are different for that gene
An organism’s phenotype
18.5
Is its physical appearance
An organism’s genotype
Is its genetic makeup
A Testcross
Allows us to determine the genotype of an organism with the dominant phenotype, but unknown genotype
Crosses an individual with the dominant phenotype with an individual that is homozygous recessive for a trait
18.6
The Dihybrid Cross
Illustrates the inheritance of two characters Helped Mendel develop the law of independent assortment
18.3
Each pair of alleles segregates independently during gamete formation
Dihybrid Cross
18.3
PRACTICE
Genotypic ratio – Phenotypic ratio –
18.3
Unfortunately… Inheritance
patterns are often more complex than predicted by simple Mendelian genetics
Types of Dominance
Complete dominance
Occurs when the phenotypes of the heterozygote and dominant homozygote are identical
Codominance
18.8
In codominance
Two dominant alleles affect the phenotype in separate, distinguishable ways
The human blood group is an example of codominance
18.9
In Incomplete Dominance
18.8
The phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties
Pleiotropy
In pleiotropy
18.10
A gene has multiple phenotypic effects
Epistasis
In epistasis
18.10
A gene at one locus alters the phenotypic expression of a gene at a second locus
BbDd
Epistas is
18.10
BbDd
Polygenic Inheritance
18.10
Many human characters vary in the population along a continuum and are called quantitative characters
One phenotype is influenced by multiple genes
Polygenic Inheritance
18.10
An additive effect of two or more genes on a single phenotype
Multifactorial characters
Are those that are influenced by both genetic and environmental factors Examples:
18.13
Heart Disease Type II Diabetes Many Cancers
Pedigree Analysis
A pedigree
18.11
Is a family tree that describes the interrelationships of parents and children across generations
Pedigree Analysis
18.11
Cystic Fibrosis
Symptoms of cystic fibrosis include
Mucus buildup in the some internal organs Abnormal absorption of nutrients in the small intestine
Sickle-Cell Disease
Sickle-cell disease
Affects one out of 400 African-Americans Is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells
Symptoms include
Physical weakness, pain, organ damage, and even paralysis
Gregor Mendel
Documented a mechanism of inheritance through his experiments with garden peas
Mendel discovered
A ratio of about three to one, purple to white flowers, in the F2 generation
EXPERIMENT True-breeding purple-flowered pea plants and white-flowered pea plants were crossed (symbolized by ×). The resulting F1 hybrids were allowed to self-pollinate or were crosspollinated with other F1 hybrids. Flower color was then observed in the F2 generation.
RESULTS Both purple-flowered plants and whiteflowered plants appeared in the F2 generation. In Mendel’s experiment, 705 plants had purple flowers, and 224 had white flowers, a ratio of about 3 purple : 1 white.
Practice Punnett Squares Ratio – Proportion –
Ratio –
18.2/18.4
Proportion –
Mendel observed the same pattern
In many other pea plant characters
Mendel’s Hypothesis
First, alternative versions of genes
Account for variationsAlleleinfor purple inherited flowers characters, which are now called alleles Locus for flower-color gene
18.1 Allele for white flowers
Homologous pair of chromosomes
Mendel’s Hypothesis
Second, for each character
18.1
An organism inherits two alleles, one from each parent A genetic locus is actually represented twice
Mendel’s Hypothesis
Third, if the two alleles at a locus differ
18.1/ 18.5
Then one, the dominant allele, determines the organism’s appearance The other allele, the recessive allele, has no noticeable effect on the organism’s appearance
Mendel’s Hypothesis
Fourth, the law of segregation
18.1
The two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes
More Genetic Vocabulary
An organism that is homozygous for a particular gene
An organism that is heterozygous for a particular gene
18.5
Has a pair of identical alleles for that gene True-breeding
Has a pair of alleles that are different for that gene
An organism’s phenotype
18.5
Is its physical appearance
An organism’s genotype
Is its genetic makeup
A Testcross
Allows us to determine the genotype of an organism with the dominant phenotype, but unknown genotype
Crosses an individual with the dominant phenotype with an individual that is homozygous recessive for a trait
18.6
The Dihybrid Cross
Illustrates the inheritance of two characters Helped Mendel develop the law of independent assortment
18.3
Each pair of alleles segregates independently during gamete formation
Dihybrid Cross
18.3
PRACTICE
Genotypic ratio – Phenotypic ratio –
18.3
Unfortunately… Inheritance
patterns are often more complex than predicted by simple Mendelian genetics
Types of Dominance
Complete dominance
Occurs when the phenotypes of the heterozygote and dominant homozygote are identical
Codominance
18.8
In codominance
Two dominant alleles affect the phenotype in separate, distinguishable ways
The human blood group is an example of codominance
18.9
In Incomplete Dominance
18.8
The phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties
Pleiotropy
In pleiotropy
18.10
A gene has multiple phenotypic effects
Epistasis
In epistasis
18.10
A gene at one locus alters the phenotypic expression of a gene at a second locus
BbDd
Epistas is
18.10
BbDd
Polygenic Inheritance
18.10
Many human characters vary in the population along a continuum and are called quantitative characters
One phenotype is influenced by multiple genes
Polygenic Inheritance
18.10
An additive effect of two or more genes on a single phenotype
Multifactorial characters
Are those that are influenced by both genetic and environmental factors Examples:
18.13
Heart Disease Type II Diabetes Many Cancers
Pedigree Analysis
A pedigree
18.11
Is a family tree that describes the interrelationships of parents and children across generations
Pedigree Analysis
18.11
Cystic Fibrosis
Symptoms of cystic fibrosis include
Mucus buildup in the some internal organs Abnormal absorption of nutrients in the small intestine
Sickle-Cell Disease
Sickle-cell disease
Affects one out of 400 African-Americans Is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells
Symptoms include
Physical weakness, pain, organ damage, and even paralysis
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