patterns of inheritance capter 10. pre-mendel’s theories blending hypothesis: when parents with...
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Pre-Mendel’s Theories
Blending Hypothesis: when parents with different traits have offspring, they will always show a blending of the traits
Spontaneous generation: Non-living matter giving rise to living matter
Homunculus: The little man inside the sperm
Trait: a variation of a particular character
Particulate hypothesis: Parents pass on to their offspring separate and distinct factors (genes) that are responsible for inherited traits
Genetics: the study of heredity
Mendel’s experiments with pea plants
Mendel’s experiments started with
True-breeding plants: when self-fertilized, a true-breeding plant produces offspring identical in appearance to itself generation after generation
Cross-fertilization, sperm from the pollen of one flower fertilizes the eggs in the flower of a different plant
Mendel’s principle of segregation
There are alternative forms of factors (genes) called alleles.
For each character, an organism has two alleles for the gene controlling that character, one from each parent. Homozygous = same alleles Heterozygous = different alleles
Principle of segregation (cont.)
When only one of the two different alleles in an heterozygous individual appears to affect the trait, that allele is called the dominant allele. The allele that does not appear to affect the trait is called the recessive allele
The two alleles for a character segregate (separate) during the formation of gametes (sex cells). Each gamete carries only one allele of each character (Principle of segregation)
Phenotype refers to the observable trait (purple flowers)
Genotype refers to the combination of alleles (PP)
Phenotypic ratio: ratio of plants with purple flowers to those with white flowers (3 purple : 1 white)
Genotypic ratio: ratio of possible combinations of alleles (1 PP : 2 Pp : 1 pp)
Phenotype refers to the observable trait (purple flowers)
Genotype refers to the combination of alleles (Pp)
• Phenotypic ratio: ratio of plants with purple flowers to those with white flowers (3 purple : 1 white)
• Genotypic ratio: ratio of possible combinations of alleles (1 PP : 2 Pp : 1 pp)
Probability and Punnett Squares
Probability: Chance of Something happening
Punnett Square: Diagramused to show the probabilityof a genetic cross
TESTCROSS
In a testcross, an individual of unknown genotype, but dominant phenotype is crossed with a homozygous recessive individual
Principle of Independent Assortment
During gamete formation in an F2 cross, a particular allele for one character can be paired with either allele of another character
Not all traits are inherited following the patterns found by Mendel in pea plants
Intermediate or incomplete inheritance
Multiple alleles – codominance
Polygenic inheritance
Environment - epigenetics
Intermediate or incomplete inheritance
The heterozygotes have a phenotype that is intermediate between the phenotypes of the two homozygotes
Multiple alleles - codominance
For many genes several alleles exist in the population.
Multiple alleles control the character of blood type in humans.
There are six possible genotypes.
The alleles IA and IB exhibit codominance, meaning that a heterozygote expresses both traits.
Polygenic inheritance
When two or more genes affect a single character
In humans, height and skin color have polygenic inheritance
The environment
Phenotype depends on environment as well as genes
Temperature affects fur color in Siamese cats
In humansNutrition affects built
Exposure to sun affects skin tone
Chromosome Theory of Inheritance
Genes are located on chromosomes
Behavior of chromosomes during meiosis and fertilization accounts for inheritance patterns – chromosomes undergo segregation and independent assortment during meiosis
Gene locus: location at which alleles of a gene reside on homologous chromosomes
Linked genes: genes that are located in the same region of a chromosome
Genetic linkage: tendency for the alleles on one chromosome to be inherited together. The closer two genes are on a chromosome, the greater the genetic linkage