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Why did Mendel use pea plants? (5)
- -Easy to grow
- -Short life span
- -Produce many offspring
- -Have traits that are observable and easy to identify
- -Can control breeding
What traits did Mendel observe?
- -Length of stem
- -Seed form (round)
- -color of seed (yellow/green)
- -Color of coat whit/gray
- -color of seed pods (green/yellow)
- -Shape of seed pod
- -position of flowers on stem
crossing of 2 different true-breds
all offspring of same variety
variant for a character
•Measures frequency of certain traits in a population
•Use randomly selected group
–project results for the whole population
Why Is Studying Humans Difficult
Humans have a long lifespan
Number of offspring tends to be small
Can’t conduct breeding experiments
- a chromosomal fragment joins a nonhomologous chromosome.
–Some translocations are reciprocal, others are not.
occurs when a chromosomal fragment reattaches to the original chromosome but in the reverse orientation.
occurs when a fragment becomes attached as an extra segment to a sister chromatid
- occurs when a chromosome fragment lacking a centromere is lost during cell division.
–This chromosome will be missing certain genes
due to three copies of chromosome 21.
–Although chromosome 21 is the smallest human chromosome, it severely alters an individual’s phenotype in specific ways
If the organism survives, aneuploidy typically leads to a distinct phenotype
•Monosomy cells have only one copy of a particular chromosome type
–Have 2n - 1 chromosomes
cells have three copies of a particular chromosome type
– Have 2n + 1 total chromosomes
Human Blood Types
- •A and B alleles are codominant
- •O allele is recessive to both A and B
More than 2 possible alleles for a gene.
– Ex: human blood types
•Alleles: A, B and O
•Phenotypes: A, B, AB, and O
The effect of one gene on the phenotypic expression of a second gene.
Genes with multiple phenotypic effect.
- –Sickle-cell anemia
The two alleles are expressed simultaneously.
–Coloration of cows
Appearance of offspring is a blend of the phenotypes of the 2 parents.
Imprinted Genes (continued)
•All the cells in an individual have genes that are imprinted
- –Both maternal and paternal imprints are present.
•During the production of gametes, both maternal and paternal imprints are “erased”.
•Then, all chromosomes are “reimprinted” according to the sex of the individual which made the gamete.
•For most autosomal genes, both alleles are expressed
•In some cases, only the allele inherited from a particular parent is expressed
–The ability to express this particular allele is determined in the gametes
•Methylation of the gene inhibits its expression
Human X-Linked Disorders
–In humans, color vision receptors in the retina are three different classes of cone cells.
•Only one type of pigment is present in each class of cone cell.
–The allele for blue-sensitive is autosomal, but the red- and green-sensitive proteins are on the X chromosome.
•Hemophilia A due to lack of clotting factor IX
•Hemophilia B due to lack of clotting factor VIII.
–Absence of protein dystrophin allows calcium to leak into muscle cells.
•Linked genes tend to be inherited together because they are located on the same chromosome
–Each chromosome has hundreds or thousands of genes.
–Genes located on the same chromosome, linked genes, tend to be inherited together because the chromosome is passed along as a unit.
–Results of crosses with linked genes deviate from those expected according to independent assortment.
Sex-Linked gene examples
•Calico and Marmelade
Thomas Hunt Morgan
- 1)Easy to raise
- 2)Short life span
- 3)Easy to distinquish contasting traits
- 4)Produce large numbers of offspring
Chromosome Theory of Inheritance
•Given that there are hundreds to thousands of traits.
–There must be more than one gene per chromosome.
–Therefore, when chromosomes segregate in meiosis, the genes of a chromosome move together.
•Genes must be linked
Walter S. Sutton
•Chromosomes and genes are both present in pairs in diploid cells.
•Homologous chromosomes separate and alleles segregate during meiosis.
•Fertilization restores the paired condition for both chromosomes and genes
Law of Independent Assortment
•Each pair of factors segregates independently of the other pairs.
•All possible combinations of factors can occur in the gametes.
- •Mendel performed cross using true-breeding plants differing in two traits.
Trait that fails to appear in the F1 generation
Trait that appears in the F1 Generation
Alternate forms of a gene
Physical appearance of an individual
Genetic makeup of an individual
Law of Segregation
•Each individual has two factors for each trait.
•The factors segregate during gamete formation.
–Each gamete contains only one factor from each pair of factors.
•Fertilization gives each new individual two factors for each trait.
•If the two factors differ, then the dominant one is fully expressed in the individual’s appearance; the recessive one has no noticeable effect on the individual’s appearance.
- cross-breeding experiments between true-breeding plants.
–Chose varieties that differed in only one trait
Second Filial generation
First filial generation