Download Heredity

Genetics Refresher
FIGURE 3.1 Gregor Johann Mendel (1822—1884)
Mendel discovered a mechanism of inheritance while conducting experiments on garden
peas at a monastery in Brunn, Austria (now Brno, Czech Republic).
The table summarizes results of Mendel’s experiments following seven characters during
crosses of the garden pea. For example, Mendel observed that in the F1 generation, the
character (or trait), seed color, occurred 6,022 yellow (dominant) : 2,001 green
(recessive), a ratio of 3.01:1, very close to his predicted ratio of 3:1.
Mendel’s experimental method
In this experiment of a cross between true breeding white- and purple-flowered plants,
Mendel pried open the surrounding petals of the purple-flowered plant and removed the
male part, thus preventing self-fertilization. Then he dusted the anther with pollen he had
selected from the white-flowered plant. The resulting seeds were planted and grew, all
producing purple flowers.
Independent segregation—single trait, flower color
Mendel’s cross of pea plants for flower color started with true breeding white-flowered
(recessive) and purple-flowered (dominant) plants. All F1 offspring of this cross were
purple-flowered, and genetically heterozygous (Pp). When these were crossed, the
resulting F2 offspring averaged 3 purple- for every 1 white-flowered plant, a 3:1
phenotypic ratio. However, the ratio of genotypes is 1:2:1 (1PP: 2Pp : 1pp).
Testcross
By just looking at a dominant phenotype, for example, this plant with purple-flowers, you would not know
if it was homozygous or heterozygous for the dominant allele. To determine its genotype, Mendel
performed a testcross. In this illustration, the dominant phenotype (unknown genotype) was crossed with
a plant known to be homozygous recessive, for example, the white-flowered plant. If all offspring are
purple (Alternative 1), then the unknown flower is homozygous dominant; if offspring are half and half,
purple and white (Alternative 2), then the unknown flower is heterozygous.
Independent assortment—multiple traits, seed shape and seed color
Mendel followed two traits together to see if they influenced each other. Vertically at left, the phenotypic
outcomes into the F2 generation are followed for seed shape—round (dominant) and wrinkled (recessive);
and seed color—yellow (dominant) and green (recessive). An underline in a genotype indicates that either
a dominant or a recessive allele is possible. If the alleles assorted or moved into gametes without affecting
each other, then the predicted ratio is 9:3:3:1, which is about what Mendel observed. Vertically at the
right, the allele combinations resulting from each successive cross are mapped, showing the genotypes.
A-T Base Pair
Adenine is a purine;
thymine is a pyrimidine.
G-C Base Pair
Guanine is a purine; Cytosine
is a pyrimidine.
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Types of Mutations
Point mutation
– Synonymous
– Transition
– Transversion
Frame-shift mutation
Stop mutation
Translocation
Fusion
Gene mutations
Normal
hemoglobin
Abnormal
hemoglobin
Chromosome mutations
Mutation Rates
Classic rate is approximately 1 X 10-6 per locus.
In other words, roughly one mutation for every million replications
Different types of mutations occur at different rates
Some Mutation Rates
Organism
Character (gene
gene)
Rate
E. coli
Lactose fermentation
lac - to lac +
2 X 10-7
Algae
Streptomycin sensitivity
Str-d to str-s
1 X 10-6
Corn
Shrunken seeds
Sh to sh
1 X 10-5
Drosophila
Yellow body
Y to y
1 X 10-4
Mouse
Dilute coat color
D to d
3 X 10-5
Human
Normal to hemophilic
3 X 10-5
Human
Normal to albino
3 X 10-5
Some mutation rates for human
genes
Types of Mutagens
Ultraviolet radiation
Ionizing radiation
Chemical mutagens
Diploidy
Most organisms have two sets of chromosomes
In humans, 22 sets of autosomes and 1 pair of sex chromosomes
Each set with about 3 billion nucleotides
Normal human
male karyotype
Normal human
female karyotype
Trisomy 21
Diploidy is important for reproduction
Haploid gametes formed by meiosis
Fertilization results in combination of two haploid gametes to
form diploid zygote
Mitosis and meiosis
In meiosis, chromosomes replicate, homologous pairs align, and each duplicated
homologue separates during cell division. Then, a second cell division separates replicated
chromosomes, and four haploid daughter cells (gametes) are produced. In mitosis,
chromosomes replicate, but one cell division separates replicated chromosomes into two
diploid daughter cells.
Meiosis results in the reduction of the
chromosome number from the diploid state
to the haploid number.
In the process of fertilization, two haploid games
unite and the diploid state is restored.
Diploidy results in a double set of chromosomes
Genetic locus – location of a gene on a chromosome
Genotype – an organism’s complement of genes
Phenotype – physical expression of the genotype
Alleles – different forms of a gene
Homozygous – describes individual with two similar alleles for a trait
Heterozygous – describes individual with two different alleles for a trait
Mendelian Ratios
Recombination may occur during meiosis
Gene linkage
Two fruit fly genes reside on the same chromosome—wing length and eye color. Because
they are located on the same chromosome, they ride together into the gametes, thereby
reducing the number of genotypes and phenotypes possible.
Multiple genes, one trait
Polygenetic inheritance is illustrated with three genes, A, B, and C; hence, six alleles, for
wheat seed color. Alleles contributing to the color are indicated by a capital letter; others
not contributing are in lower case. In each generation, the six alleles are shown by circles—
solid if contributing and open if not adding to seed color. In the F2 generation, the
phenotypes expressed are additive, producing a continuous range of seed color. If graphed
by frequency, they form a bell-shaped curve.
FIGURE 3.13 Polygenic Trait in Humans—Height
Aligned by height, the students show a range of continuous phenotypic variation, with most
in the middle.
FIGURE 3.11 Multiple Alleles, One Trait
In humans, the gene that controls blood type has three different alleles: A, B, and O,
resulting in four different blood phenotypes: A, B, AB, O, specifically type A (A/A or A/O);
type B (B/B or B/O); type AB (A/B); and type O (O/O).
Mendelian inheritance
versus Blending