Chapter11- Mendels
Transcript of Chapter11- Mendels
Observing Patterns in Inherited Traits
Chapter 11
Impacts, Issues:The Color of Skin
Like most human traits, skin color has a genetic basis; more than 100 gene products affect the synthesis and deposition of melanins
11.1 Mendel, Pea Plants, and Inheritance Patterns
Recurring inheritance patterns are observable outcomes of sexual reproduction
Before the discovery of genes, it was thought that inherited traits resulted from a blend of parental characters
Mendel’s Experimental Approach
Mendel was a monk with training in plant breeding and mathematics
He studied the garden pea (Pisum sativum), which breeds true for a number of traits
Garden Pea Plant: Self Fertilization and Cross-Fertilization
Fig. 11-3, p. 170
carpel anther
A Garden pea flower, cut in half. Sperm form in pollen grains, which originate in male floral parts (anthers). Eggs develop, fertilization takes place, and seeds mature in female floral parts (carpels).
B Pollen from a plant that breeds true for purple flowers is brushed onto a floral bud of a plant that breeds true for white flowers. The white flower had its anthers snipped off. Artificial pollination is one way to ensure that a plant will not self-fertilize.
C Later, seeds develop inside pods of the cross-fertilized plant. An embryo in each seed develops into a mature pea plant.
D Each new plant’s flower color is indirect but observable evidence that hereditary material has been transmitted from the parent plants.
Animation: Crossing garden pea plants
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)
F1: First generation offspring of parents
F2: Second generation offspring of parents
11.1 Key ConceptsWhere 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
11.2 Mendel’s Law of Segregation
Garden pea plants inherit two “units” of information for a trait, one from each parent
Testcrosses
Testcross• A method of determining if an individual is
heterozygous or homozygous dominant• An individual with unknown genotype is crossed
with one that is homozygous recessive (AA x aa) or (Aa x aa)
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
Fig. 11-5, p. 172
homozygous dominant parent
homozygous recessive parent
(chromosomes duplicated before
meiosis)
meiosis I
meiosis II
(gametes) (gametes)
fertilization produces heterozygous offspring
Fig. 11-5, p. 172
homozygous dominant parent
homozygous recessive parent
(chromosomes duplicated before
meiosis)
meiosis I
meiosis II
(gametes) (gametes)
fertilization produces heterozygous offspring
Stepped Art
Mendel’s Monohybrid Experiments
Fig. 11-6, p. 172
Trait Studied
Dominant Form
Recessive Form
F2 Dominant-to- Recessive Ratio
Seed shape 5,474 round 1,850 wrinkled 2.98 to 1
Seed color 6,022 yellow 2,001 green 3.01 to 1
Pod shape 882 inflated 299 wrinkled 2.95 to 1
Pod color 428 green 152 yellow 2.82 to 1
Flower color 705 purple 224 white 3.15 to 1
Flower position 651 along stem 207 at tip 3.14 to 1
Stem length 787 tall 277 dwarf 2.84 to 1
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
Fig. 11-7, p. 173
Fig. 11-7a, p. 173
female gametes
A a A a A a
A
a
A A A
Aa
A
AA
Aa
mal
e g
amet
es
a aa a Aa aa a
Aa
aa a
Aa
aa
A From left to right, step-by-step construction of a Punnett square. Circles signify gametes, and letters signify alleles: A is dominant; a is recessive. The genotypes of the resulting offspring are inside the squares.
Fig. 11-7b, p. 173
aaF1 offspring
True-breeding homozygous recessive parent plant
a aAa Aa
A Aa Aa
AA
A Aa Aa
True-breeding homozygous dominant parent plant
Aa Aa
B A cross between two plants that breed true for different forms of a trait produces F1 offspring that are identically heterozygous.
Fig. 11-7c, p. 173
AaF2 offspring
Heterozygous F1 offspring
A a
A AA Aa
AA Aa
a Aa aaAa
aaHeterozygous F1 offspring Aa
C A cross between the F1 offspring is the monohybrid experiment. The phenotype ratio of F2 offspring in this example is 3:1 (3 purple to 1 white).
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
11.2 Key ConceptsInsights 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
11.3 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
Fig. 11-8, p. 174
One of two possible alignments The only other possible alignment
a Chromosome alignments at metaphase I:
A A a a A A a a
B B b b b b B B
b The resulting alignments at metaphase II:
A A a a A A a a
B B b b b b B B
c Possible combinations of alleles in gametes:
B A A B b a a b b A A b B a a B
AB ab Ab aB
Fig. 11-8, p. 174
One of two possible alignments
a Chromosome alignments at metaphase I:
A A a a
B B b b
The only other possible alignment
A A a a
b b B B
b The resulting alignments at metaphase II:
A A a a
B B b b
A A a a
b b B B
c Possible combinations of alleles in gametes:
B A A B b a a b
AB ab
b A A b B a a B
Ab aB
Stepped Art
Mendel’s Dihybrid Experiments
Fig. 11-9a, p. 175
Fig. 11-9a, p. 175
P generation
parent plant homozygous
for purple flowers
and long stems
parent plant homozygous
for white flowers
and short stems
A Meiosis in homozygous individuals results in one kind of gamete. AABB aabb
B A cross between plants homozygous for two different traits yields one possible combination of gametes:
AB x ab
Fig. 11-9b, p. 175
Fig. 11-9b, p. 175
F2 generation
AaBb AaBb AaBbAll F1 offspring are AaBb, with purple flowers and tall stems.
C Meiosis in AaBb dihybrid plants results in four kinds of gametes:
AB Ab aB ab
These gametes can meet up in one of 16 possible wayswhen the dihybrids are crossed (AaBb X AaBb):
F1 generation
Fig. 11-9c, p. 175
Fig. 11-9c, p. 175
AB Ab aB ab
AB AABB AABb AaBB AaBb
Ab AABb AAbb AaBb Aabb
aB AaBB AaBb aaBB aaBb
ab AaBb Aabb aaBb aabb
D Out of 16 possible genetic outcomes of this dihybrid cross, 9 will result in plants that are purple-flowered and tall; 3, purple-flowered and short; 3, white-flowered and tall; and 1, white-flowered and short. The ratio of phenotypes of this dihybrid cross is 9:3:3:1.
Animation: Dihybrid cross
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
11.3 Key ConceptsInsights from Dihybrid Experiments
Some experiments yielded evidence of independent assortment: Genes are typically distributed into gametes independently of other genes
11.4 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
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
Fig. 11-10, p. 176
AA BB
Genotypes: AO AB BO OO
Phenotypes (Blood type): A AB B O
or or
Animation: Codominance: ABO blood types
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
Fig. 11-11a, p. 176
Fig. 11-11a, p. 176
homozygous parent (RR)
xhomozygous parent (rr)
heterozygous F1 offspring (Rr)
A Cross a red-flowered with a white-flowered plant, and all of the F1 offspring will be pink.
Fig. 11-11b, p. 176
Fig. 11-11b, p. 176
R r
B Cross two F1 plants, and the three phenotypes of the F2 offspring will occur in a 1:2 :1 ratio:
R
RR Rr
r
Rr rr
Epistasis
Epistasis• Two or more gene products influence a trait• Typically, one gene product suppresses the effect
of another
Example: Coat color in dogs• Alleles B and b designate colors (black or brown)• Two recessive alleles ee suppress color
Epistasis in Coat Colors
Fig. 11-13a, p. 177
EB Eb eB eb
EB black black blackEEBB EEBb EeBB
blackEeBb
Eb black chocolate black chocolateEEBb EEbb EeBb Eebb
eB black black yellow yellowEeBB EeBb eeBB eeBb
EeBb Eebb eeBbeb black chocolate yellow yellow
eebb
Epistasis in Chicken Combs
Pleiotropy
Pleiotropy• One gene product
influences two or more traits
• Example: Some tall, thin athletes have Marfan syndrome, a potentially fatal genetic disorder
11.5 Linkage Groups
The farther apart two genes are on a chromosome, the more often crossing over occurs between them
Linkage group• All genes on one chromosome • Linked genes are very close together; crossing
over rarely occurs between them
Linkage and Crossing Over
Fig. 11-15, p. 178
Parental generation
AC
F1 offspring All AaCc
meiosis, gamete formation
Gametes
Most gametes have parental genotypes
A smaller number have recombinant genotypes
ac
X
Animation: Crossover review
The Distance Between Genes
The probability that a crossover event will separate alleles of two genes is proportional to the distance between those genes
11.6 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
Animation: Coat color in the Himalayan rabbit
Effects of Altitude on Gene Expression
Fig. 11-17, p. 179
a Mature cutting at high elevation (3,060 meters above sea level)
60
Hei
gh
t (c
enti
met
ers)
0
b Mature cutting at mid-elevation (1,400 meters above sea level)
60
Hei
gh
t (c
enti
met
ers)
0
c Mature cutting at low elevation (30 meters above sea level)
60
Hei
gh
t (c
enti
met
ers)
0
Effects of Predation on Gene Expression
Predators of daphnias emit chemicals that trigger a different phenotype
Fig. 11-18a, p. 179
Fig. 11-18b, p. 179
11.7 Complex Variations in Traits
Individuals of most species vary in some of their shared traits
Many traits (such as eye color) show a continuous range of variation
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
Bell curve• When continuous phenotypes are divided into
measurable categories and plotted as a bar chart, they form a bell-shaped curve
Continuous Variation and the Bell Curve
Fig. 11-19a, p. 180
Fig. 11-19b, p. 180
Fig. 11-19c, p. 180
Animation: Continuous variation in height
Regarding the Unexpected Phenotype
Phenotype results from complex interactions among gene products and the environment• Enzymes and other gene products control steps
of most metabolic pathways• Mutations, interactions among genes, and
environmental conditions may affect one or more steps
11.4-11.7 Key ConceptsVariations 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• Multiple genes may influence a trait; some genes
influence many traits• The environments also influences gene
expression
Animation: Testcross
Animation: Coat color in Labrador retrievers
Animation: Comb shape in chickens
Animation: F2 ratios interaction
Animation: Genetic terms
Animation: Incomplete dominance
Animation: Monohybrid cross
Animation: Pleiotropic effects of Marfan syndrome
Video: Genetics of skin color