Anthropology Mini Essays
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lecture_5_genetics.pdf
Lecture 5: Genetics
Today’s Questions
• What is Mendelian, or simple, inheritance?
• What are the principles of Mendelian Inheritance?
• What is polygenic inheritance?
• What is epigenetics?
Gregor Mendel
1822-1884 Czech Monk
Mendel’s Pea Plants
Important Terms
• Gene
• Allele
• Genotype
• Phenotype
• Dominant
• Recessive
• Heterozygous
• Homozygous
http://www.zo.utexas.edu/faculty/sjasper/images/14.3.gif
Important Terms
• Gene—a specific sequence of nucleotide base pairs at a specific location on a chromosome
• Alleles—alternative forms of a gene at the same locus on a homologous pair of chromosomes
• Genotype—the genetic make-up of an individual
• Phenotype—the observable physical characteristics of an organism; the expression of the genotype
Important Terms
• Dominant—a trait governed by an allele that is expressed in the presence of another allele; dominant alleles prevent the expression of recessive alleles in heterozygotes
• Recessive—a trait that isn’t expressed in heterozygotes; for a recessive trait to be expressed, an individual must have two copies of it—homozygous
Important Terms
• Heterozygous—having different alleles at the same locus on members of a pair of chromosomes
• Homozygous—having the same allele at the same locus on both members of a pair of chromosomes
• https://www.youtube.com/watch?v=Mehz7tC xjSE
The First Experiment
• P1 Generation:
– Tall Plants x short
• F1 Generation
– All Tall Plants
• F2 Generation
– ¾ Tall, ¼ short
Mendel’s Principles of Inheritance
• Discrete Units of Inheritance
– Genes (Mendel did not know about genes)
• Principle of Segregation
– Genes occur in pairs—one from each parent
– Meiosis—the members of pairs separate, one each going into a gamete (haploid sex cells) F
– Fertilization brings two genes back together
Mendel’s Principles of Inheritance
• Principle of Dominance and Recessiveness
– Some traits are expressed no matter what
• Homozygous dominant
• Heterozygous dominant
– Some traits are only expressed when there are two copies of the same allele at that locus
• Homozygous recessive
• Principle of Independent Assortment
Reproduction and Inheritance
• Punnett Square
– Offspring ratios
– Probability of inheriting alleles from each parent
– Based on possible gamete allele combinations
– Relies on principles of segregation and independent assortment
Predicting Offspring
• During meiosis, alleles separate:
Aa AaCc
Oogonia/Spermatogonia
A a AC Ac aC ac
possible gametes
Predicting Offspring
• If an individual’s genotype for a gene is Aa, what distinct gametes can be produced? Remember, gametes in humans are haploid.
Predicting Offspring
• If an individual’s genotype for a gene is Aa, what distinct gametes can be produced? Remember, gametes in humans are haploid.
• (A) and (a)
• 2 distinct types, or 21
– 2 alleles, for one gene means the number of possible gamete genotypes equals 21, or 2
Predicting Offspring
• If an individual’s genotype for two genes is AaBb, what distinct gametes can be produced?
Predicting Offspring
• If an individual’s genotype for two genes is AaBb, what distinct gametes can be produced?
• (AB), (Ab), (aB), and (ab)
• Now, it is 4 distinct types, or 22
– 2 alleles per gene, for two genes means possible gametes = 2 x 2 = 4
T T
t
t
P1 = TT x tt T = Tall t = short
Parental Generation Gametes (purebred plants)
T T
t Tt Tt
t Tt Tt
Offspring (F1) phenotypic ratio = Genotypic Ratio =
T T
t Tt Tt
t Tt Tt
Offspring (F1) phenotypic ratio = 4:0. They are all tall. Genotypic Ratio = 4:0. They are all Tt.
T t
T
t
F2: Phenotypic Ratio = Genotypic Ratio =
Then he self-fertilized F1…
Tt x Tt
T t
T TT Tt
t Tt tt
F2: Phenotypic Ratio = Genotypic Ratio =
F1 Self-fertilization
T t
T TT Tt
t Tt tt
F2: Phenotypic Ratio = 3Tall:1Short Genotypic Ratio =1TT:2Tt:1tt
F1 Self-fertilization
T t
T TT Tt
t Tt tt
F1 Self-fertilization
Homozygous dominant = Heterozygous dominant = Homozygous recessive =
T t
T TT Tt
t Tt tt
F1 Self-fertilization
Homozygous dominant = TT Heterozygous dominant = Homozygous recessive =
T t
T TT Tt
t Tt tt
F1 Self-fertilization
Homozygous dominant = TT Heterozygous dominant = Tt Homozygous recessive =
T t
T TT Tt
t Tt tt
F1 Self-fertilization
Homozygous dominant = TT Heterozygous dominant = Tt Homozygous recessive = tt
The First Experiment
• This illustrates the first 3 of Mendel’s Principles
– Units of Inheritance
– Segregation
– Dominance and Recessiveness
Principle of Independent Assortment
• The distribution of one pair of alleles in gametes does not influence the distribution of another pair. – Meiosis tells us this
– Random assortment of genes into gametes
– Which side of the dividing cell a particular chromosome goes into is random
– Except in the case of genes that are close together on the same chromosome • They may cross over together leading to linkages
Two Traits
Two Traits
• Parental Genotype
– TTYY
– ttyy
• Parental Gametes?
Two Traits
• Parental Genotype
– TTYY
– ttyy
• Parental Gametes?
– TY, TY, TY, TY
– ty, ty, ty, ty
TY Ty tY ty
TY
Ty
tY
ty
Self fertilization of F1 for two traits
TY Ty tY ty
TY TT TT Tt Tt
Ty TT TT Tt Tt
tY Tt Tt tt tt
ty Tt Tt tt tt
Self fertilization of F1 for two traits
TY Ty tY ty
TY TTYY TTYy TtYY TtYy
Ty TTYy TTyy TtYy Ttyy
tY TtYY TtYy ttYY ttYy
ty TtYy Ttyy ttYy ttyy
Tall/Yellow: 9 short/Yellow: 3 Phenotypic ratio (F2) Tall/green: 3 short/green: 1 9:3:3:1
Self fertilization of F1 for two traits
Principle of Independent Assortment
Mendelian Traits in Humans
Polydactyly—six fingers
And a cat…
My, what big feet you have…
More Mendelian Traits In Humans
• Hair Whorl
• Ear Lobes—attached or free
Exceptions to Mendelian Inheritance
• Incomplete Dominance—when the dominant allele does not fully mask the expression of the recessive allele
– Heterozygotes show intermediate expression of a trait
– Ex. Sickle-cell trait
Hemoglobin
• There are two alleles for hemoglobin – HbA = normal hemoglobin
– HbS = sickle hemoglobin
• There are three genotypes and three phenotypes – HbA HbA = produce all normal hemoglobin
– HbA HbS = produce mostly normal hemoglobin but some sickling hemoglobin (sickle-cell trait)
– HbS HbS = sickle-cell anemia
Exceptions to Mendelian INheritance
• Co-Dominance—when there are two or more dominant alleles that are equally expressed in the phenotype
Blood Type and Co-Dominance
Mendelian and Polygenic Inheritance
Mendelian and Polygenic Inheritance
• Mendelian traits are influenced by the action of only one gene
– Mendelian traits are discontinuously distributed, i.e. they are categorical.
• Polygenic traits are influenced by the action of more than one gene
– Polygenic traits are continuously distributed
– Most human traits are polygenic
Skin Color and Polygenic Inheritance
http://anthro.palomar.edu/adapt/images/map_of_skin_color_distribution.gif
Skin Color and Polygenic Inheritance
• Skin color
– Melanin—protein; pigment that influences skin color in humans
– At least 6 different genes influence melanin production
– Each of those genes has at least 2 different alleles
– Skin color varies continuously
Genetic and Environmental Factors
• For polygenic traits...
– The genotype limits but does not determine the phenotype
– The genotype interacts with the environment to determine the phenotype
• For Mendelian traits...
– Less likely to be influenced by environment
Genes + Environment = Phenotype
Height Genes
Explain 60-80% variation in height
At least 180 genes
Maternal Nutrition
Environmental Pollutants
Childhood Nutrition
Other?
Epigenetics
• “the set of chemical modifications surrounding and attaching to our genetic material that change the way genes are switched on or off, but don’t alter the genes themselves” (Carey, 2013, p. 29)
Epigenetics
http://thesocietypages.org/sociologylens/files/2011/03/Epigenetic_mechanisms.jpg
Dutch Hunger Winter
• Maternal Nutrition and Life History Outcomes
– Malnourished late in pregnancysmall infantslower rates of obesity
– Malnourished early in pregnancynormal birth weightshigher rates of obesity and mental health issues
http://www.precisionnutrition.com/wordpress/wp-content/uploads/2009/12/47-Hunger-Winter-Boy.jpg
Genes + Environment = Phenotype
Height Genes
Explain 60-80% variation in height
At least 180 genes
Maternal Nutrition
Environmental Pollutants
Childhood Nutrition
Other?
Epigenetic Factors
