Download Genetics - De Anza

Genetics
Early Ideas about Heredity
 People knew that sperm and eggs transmitted
information about traits
 Blending theory – mother and father’s traits blended
together
 Problem:
• Would expect variation to disappear
• Variation in traits persists (Example: freckles show up
in unfreckled parents’ offspring!)
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Mendel’s Experimental Approach
 Mendel was a monk with training in plant breeding and
mathematics – considered to be the founder of modern
genetics! (1822 – 1884)
 He studied the garden pea (Pisum sativum), which
breeds true for a number of traits
The Garden Pea Plant
 Self-pollinating
 True breeding (different alleles not
normally introduced)
 Can be experimentally crosspollinated – excellent for genetic
manipulation!!!
 Small, easy to grow, can self-fertilize,
easily distinguishable traits!
Terms Used in Modern Genetics
 Genes
• Heritable units of information about traits
• Parents transmit genes to offspring
• Each gene has a specific locus on a
chromosome
 Diploid cells (chromosome number 2n) have
pairs of genes on homologous chromosomes
Terms Used in Modern Genetics
 A mutation is a permanent change in a gene
• May cause a trait to change
• Alleles are different molecular forms of a gene
 A hybrid has nonidentical alleles for a trait
• Offspring of a cross between two individuals that
breed true for different forms of a trait are hybrids
Terms Used in Modern Genetics
 An individual with nonidentical alleles of a gene
is heterozygous for that gene
 An individual with identical alleles of a gene is
homozygous for that gene
Terms Used in Modern Genetics
 An allele is dominant if its effect masks the
effect of a recessive allele paired with it
• Capital letters (A) signify dominant alleles;
lowercase letters (a) signify recessive alleles
• Homozygous dominant (AA)
• Homozygous recessive (aa)
• Heterozygous (Aa)
Terms Used in Modern Genetics
 Gene expression
• The process by which information in a gene is
converted to a structural or functional part of a
cell or body
• Expressed genes determine traits
Terms Used in Modern Genetics
 Genotype
• The particular alleles an individual carries
 Phenotype
• An individual’s observable traits
Terms Used in Modern Genetics
 P stands for parents, F for filial (offspring)
• AA x aa
 F1: First generation offspring of parents
• Aa x Aa
 F2: Second generation offspring of parents
• AA, Aa, Aa, aa
Key Concepts
Where Modern Genetics Started
 Gregor Mendel gathered the first experimental
evidence of the genetic basis of inheritance
 His meticulous work gave him clues that
heritable traits are specified in units
 The units, which are distributed into gametes in
predictable patterns, were later identified as
genes
Mendel’s Law of Segregation
 Garden pea plants inherit two “units” of
information for a trait, one from each parent
Monohybrid Experiments
 Monohybrid experiments
• Testcrosses that check for a dominance
relationship between two alleles at a single locus
• May be crosses between true breeding
(homozygous) individuals (AA x aa), or between
identical heterozygotes (Aa x Aa)
Mendel’s Monohybrid Experiments
 Mendel used monohybrid experiments to find
dominance relationships among pea plant traits
• When he crossed plants that bred true for white
flowers with plants that bred true for purple
flowers, all F1 plants had purple flowers
• When he crossed two F1 plants, ¾ of the F2
plants had purple flowers, ¼ had white flowers
Segregation of Alleles at a Gene Locus
Mendel’s Monohybrid Experiments
Calculating Probabilities
 Probability
• A measure of the chance that a particular
outcome will occur
 Punnett square
• A grid used to calculate the probability of
genotypes and phenotypes in offspring
Construction of a Punnett Square
Phenotype Ratios
in a Monohybrid Experiment
Phenotype Ratios
in a Monohybrid Experiment
Mendel’s Law of Segregation
 Mendel observed a phenotype ratio of 3:1 in the
F2 offspring of his monohybrid crosses
• Consistent with the probability of the aa genotype
in the offspring of a heterozygous cross (Aa x Aa)
 This is the basis of Mendel’s law of segregation
• Diploid cells have pairs of genes on pairs of
homologous chromosomes
• The two genes of each pair separate during
meiosis, and end up in different gametes
Key Concepts
Insights from Monohybrid Experiments
 Some experiments yielded evidence of gene
segregation: When one chromosome separates
from its homologous partner during meiosis, the
alleles on those chromosomes also separate
and end up in different gametes
http://www.youtube.com/v/F3AKldl6JZg - practice
with Punnett Squares!!! To be done on your own or
with me in office hours!!!
Mendel’s Law
of Independent Assortment
 Mendel’s law of independent assortment
• Many genes are sorted into gametes
independently of other genes
Dihybrid Experiments
 Dihybrid experiments
• Tests for dominance relationships between
alleles at two loci
• Individuals that breed true for two different traits
are crossed (AABB x aabb)
• F2 phenotype ratio is 9:3:3:1 (four phenotypes)
• Individually, each dominant trait has an F2 ratio of
3:1 – inheritance of one trait does not affect
inheritance of the other
Independent Assortment at Meiosis
Mendel’s Dihybrid Experiments
Mendel’s Law of Independent Assortment
 Mendel’s dihybrid experiments showed that
“units” specifying one trait segregated into
gametes separately from “units” for other traits
 Exception: Genes that have loci very close to
one another on a chromosome tend to stay
together during meiosis
Key Concepts
Insights from Dihybrid Experiments
 Some experiments yielded evidence of
independent assortment: Genes are typically
distributed into gametes independently of other
genes
Beyond Simple Dominance
 Mendel focused on traits based on clearly
dominant and recessive alleles; however, the
expression patterns of genes for some traits are
not as straightforward
Continuous Variation
 Continuous variation
• Traits with a range of small differences
• The more factors that influence a trait,
the more continuous the distribution of
phenotype
Incomplete Dominance
 Incomplete dominance
• One allele is not fully dominant over its partner
• The heterozygote’s phenotype is somewhere
between the two homozygotes, resulting in a
1:2:1 phenotype ratio in F2 offspring
 Example: Snapdragon color
• RR is red
• Rr is pink
• rr is white
Incomplete Dominance in Snapdragons
Genes and the Environment
 Expression of some genes is affected by
environmental factors such as temperature,
altitude, or chemical exposure
 The result may be variation in traits
Effects of Temperature
on Gene Expression
 Enzyme tyrosinase, works at low temperatures
Codominance in ABO Blood Types
 Codominance
• Two nonidentical alleles of a gene are both fully
expressed in heterozygotes, so neither is
dominant or recessive
• May occur in multiple allele systems
 Multiple allele systems
• Genes with three or more alleles in a population
• Example: ABO blood types
Codominance in ABO Blood Types
ABO Blood Type:
Glycolipids on Red Cells
 Type A - Glycolipid A on cell surface
 Type B - Glycolipid B on cell surface
 Type AB - Both glyocolipids A & B
 Type O - Neither glyocolipid A nor B
Regarding the Unexpected Phenotype
 Phenotype results from complex interactions among
gene products and the environment – identical twins!!!
• Enzymes and other gene products control steps of
most metabolic pathways
• Mutations, interactions among genes, and
environmental conditions may affect one or more steps
Key Concepts
Variations on Mendel’s Theme
 Not all traits appear in Mendelian inheritance
patterns
• An allele may be partly dominant over a
nonidentical partner, or codominant with it
• The environments also influences gene
expression
Chromosomal Abnormalities – some examples
 Errors sometimes occur during meiosis
• Down Syndrome – extra copy of Chromosome #21
• Klinefelter Syndrome (XXY male) – sterile male with
female characteristics and diminished mental capacity
(extra X). Normally XX and XY
• Turner Syndrome (XO female) – sterile female with
webbed neck and diminished stature. Missing all or
part of 2nd X chromosome