review
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Review. Mendel’s “rules of the game”. 1) Genes occur in pairs - Genetic characteristics are controlled by genes that exist in pairs called alleles. - PowerPoint PPT PresentationTRANSCRIPT
Review
Mendel’s “rules of the game”
3) Segregation- during formation of the gametes, the paired alleles separate or segregate randomly.
1) Genes occur in pairs - Genetic characteristics are controlled by genes that exist in pairs called alleles.
2) Dominance/Recessiveness- When two unlike alleles responsible for a single character are present in a single individual, one is dominant (expressed) to the other which is said to be recessive (silent).
Yellow seeded plants in the F2 are predicted to have either GG or Gg genotypes.
Is there a way to distinguish the genotype?
Question
Test Cross
The organism of dominant phenotype but unknown genotype is crossed to a homozygous recessive individual.
+o oXGg gg
g
G
g
gPossible sperm cells
Possible egg cells
gg
GgGg
gg
GG?
Test Cross
If the offspring show 1:1 ratio of dominant:recessive phenotypes, the parent in question must have been heterozygous (Gg).
+o oXGg ggGG
? g
G
G
gPossible sperm cells
Possible egg cells
GgGg
GgGg
Test Cross
If the offspring show only the dominant phenotype, the parent in question must have been a homozygous dominant (GG) individual.
Discovering genes via Mutant analysis
• Generating mutants
– Chemical mutagenesis (EMS)
– base transition, point mutation
– Radiation
– deletions
– Transposons/ T-DNA tags
– insertion/deletions (indels)
ID the phenotype
ID the gene (DNA sequence)
ID the physiological, developmental,molecular differences
Forward Genetics- from phenotype to gene
And observing segregation ratios
• Planned crosses and Punnett squares, • Pedigree analysis– Mutant or polymorphism analysis
• Autosomal dominant/recessive
• Sex-linked genes
If A represents a gene that displays autosomal dominant/recessive inheritance. The genotype of I-1 must be:
a. AAb. Aac. Aad. Insufficient data to tell
The genotype of I-1 must be:
a. AAb. Aac. Aad. Insufficient data to tell
If the DNA seen on gel 1 is from a pea plant heterozygous for “R” and “r” at the SBE1 locus, the DNA in lane 2 is most likely from a pea plant
a. Homozygous for the “R” alleleb. Homozygous for the “r” allelec. Unable to make functional starch branching enzyme 1d. That is making twice the normal amount of starch
branching enzyme 1
If the DNA seen on gel 1 is from a pea plant heterozygous for “R” and “r” at the SBE1 locus, the DNA in gel 2 is most likely from a pea plant
a. Homozygous for the “R” alleleb. Homozygous for the “r” allelec. Unable to make functional starch branching enzyme 1d. That is making twice the normal amount of starch
branching enzyme 1
Rr
1 2 3
Reverse Genetics- from gene to phenotype
• ID a gene
• Mutate, knockout, over-express the gene
• Analyze the morphological, physiology, developmental effects (the phenotype).
Independent AssortmentChapter 3
Dihybrid cross
- crosses between individuals that differ in two traits.
Xyellow, round
P1 cross
F1
green, wrinkled
yellow, round
Xgreen, round
P1 cross
yellow, wrinkled
Mendel’s dihybrid crosses
Self-pollination of the F1
Xyellow, round yellow, round
9/16 yellow, round
1/16 green, wrinkled
3/16 green, round
3/16 yellow, wrinkled
F1
F2
Xyellow (Gg), round (Ww) yellow (Gg), round (Ww)
GW Gw gW gw+o
o
GW
Gw
gW
gw
GGWW
GgWw
GgWwGgWW
GgWw
GgWW
GgWwGGWw
GGWw
ggWw
ggWwggWW
ggwwGgww
GGww Ggww
Generation
F2
Mendel’s Second Principle of Inheritance
Independent Assortment - during gamete formation, segregating pairs of unit factors assort independently of each other.
Mendel’s dihybrid ratio 9:3:3:1
But, what if
Xyellow (Gg), round (Ww) yellow (Gg), round (Ww)
Xyellow (Gg), round (Ww) yellow (Gg), round (Ww)
+oo
GWGGWW
GgWw
GgWwGgWw
GgWw
GgWw
GgWwGGWW
GGWW
ggww
ggwwggww
ggwwGgWw
GGWW GgWw
Generation
F2
GW GW gw gw
gw
gw
GW
Phenotypic Ratio
9Yellow, round
1green, wrinkle
3green, round
3Yellow, wrinkle
Expected F2
12Yellow, round
4green, wrinkle
0green, round
0Yellow, wrinkle
Resulting F2
Xyellow (Gg), round (Ww) yellow (Gg), round (Ww)
Chromosomal Basis of Inheritance
Meiosis (the prelude to
sexual reproduction)
For sexual reproduction to occur, chromosomes must be duplicated and divided between the gametes.
Meiosis I
Meiosis II
Independent Assortment
- during gamete formation, segregating pairs of chromosomes (not genes) assort independently of each other.
Mitosis takes place in
a.Haploid cells onlyb.Diploid cells onlyc.Haploid or diploid cellsd.Bacterial cellse.None of the above
Mitosis takes place in
a.Haploid cells onlyb.Diploid cells onlyc.Haploid or diploid cellsd.Bacterial cellse.None of the above
Meiosis takes place in
a. Haploid cells onlyb. Diploid cells onlyc. Haploid or diploid cellsd. Somatic cellse. None of the above
Meiosis takes place in
a. Haploid cells onlyb. Diploid cells onlyc. Haploid or diploid cellsd. Somatic cellse. None of the above
Polygenic Traits (Quantitative trait loci, QTLs)
Observation: Wheat kernel color is a continuum from
white dark red
Experiment 1
+o oX P1
F1 - all light red
Experiment 2
+o oXF1
F2
1 6 15 20 15 6 1
Frequency Diagram
Wheat Kernel Color
3 loci (polygenic), 6 different alleles
F2
seed color
Dark white red
# of dominantalleles
6 5 4 3 2 1 0
Continuous variation is determined by two or more genes.
These are polygenic or quantitative traits.
Cytoplasmic Segregation(Non-Mendelian Genetics)
• Chloroplast DNA (cpDNA)
• Mitochondrial DNA (mt DNA)
Cytoplasmic Segregation
Test Cross ?
• Reciprocal crosses– Who’s the pollen donor?
A human disease associated with dysfunction of mitochondria, which results from a mutation in a single autosomal gene locus in nuclear DNA, is most likely to:
a.Be inherited from the mother, because mitochondria are not inherited from the fatherb.Show heteroplasmy in the progeny of affected individuals, depending on chance events during meiosisc.Show a non-Mendelian inheritance pattern, because the number of mitochondria varies from cell to celld.Show a Mendelian inheritance patterne.None of the above
A human disease associated with dysfunction of mitochondria, which results from a mutation in a single autosomal gene locus in nuclear DNA, is most likely to:
a.Be inherited from the mother, because mitochondria are not inherited from the fatherb.Show heteroplasmy (a mix of WT and mutant mitochondria) in the progeny of affected individuals, depending on chance events during meiosisc.Show a non-Mendelian inheritance pattern, because the number of mitochondria varies from cell to celld.Show a Mendelian inheritance patterne.None of the above