Midterm 2 (2.3) Flashcards
Stamens
Produce pollen
Male parts
Carpels
Bears eggs
Female part
True-breeding plants
Plants self-pollinated over many generations
Produced only the same variety as the parent plant
Hybridization
Crossing of two true-breeding varieties
Dominant allele
Determines organism’s appearance
Recessive allele
Has no noticeable effect on organism’s appearance
Law of segregation
2 alleles for a character segregate during gamete formation
- End up in different gametes
Egg or sperm gets only ONE of two alleles present in somatic cells
If individual is true-breeding
– All gametes have identical alleles for particular character
If different alleles present
– 50% gametes get dominant allele
– 50% gametes get recessive allele
Homozygous
2 identical alleles for a gene
All gametes contain same allele
Heterozygous
2 different allele for a gene
1/2 gametes will have ones allele, 1/2 gametes will have the other
Phenotype
Appearance, observable traits
Genotype
Genetic makeup
Monohybrid
Heterozygous for one character followed in a cross
All F1 progeny from crosses of true-breeding parents are monohybrids
Monohybrid cross
Cross between heterozygous for one character
Dihybrids
Individuals heterozygous for two characters followed in a cross
YyRr
F1 plants formed by crossing two true-breeding pea varieties that differ in both of these characters
Dihybrid cross
Cross between F1 dihybrids
Are two characters transmitted as a package or independently?
Law of independent assortment
Each pair of alleles segregates independently of each other pair of alleles during gamete formation
Applies only to genes on different homologous chromosomes
Or far apart on same chromosome
Complete dominance
Phenotype of heterozygote and dominant homozygote indistinguishable
Incomplete dominance
Phenotype of heterozygote is intermediate between phenotypes of individuals homozygous for either allele
Codominance
2 alleles each affect phenotype in separate ways
Phenotypes of both alleles exhibited in heterozygote
Pleiotropy
Ability of a single gene to have multiple phenotypic effects
Example: gene that determines flower color in garden pea also affects colour of coating on outer surface of seed
Epistasis
Phenotypic expression of gene at one locus alters that of gene at second locus
Example: Labrador retrievers
– Black coat (B) dominant to brown coat (b)
– Separate gene determines whether pigment will be deposited in hair or not
– Dominant allele (E) results in black or brown coat
Polygenic inheritance
Additive effect of ≥2 genes on single phenotypic character
Converse of pleiotropy
Pedigree
Diagram of family tree showing occurrence of heritable characters in parents and offspring over multiple generations
Wildtype
Phenotype most commonly observed in natural populations
Cystic fibrosis
Normal (wildtype) allele gene codes for membrane protein involved in Cl- transport in and out of cells
Homozygous recessive
- Ultimate result is thicker mucus coating certain cells, builds up in pancreas, lungs, digestive tract
Tay-Sachs disease
Neurodegenerative disease
Sickle-cell anemia
Affects 1/400 African-Americans
1 amino acid substituted for another in hemoglobin
Change in amino acid caused by point mutation
Heterozygotes may have some symptoms
– Wild-type allele not completely dominant
May persist in population because it reduces risk of malaria
– Advantage to heterozygotes
Dominantly inherited disorders
Achondroplasia
- From of dwarfism
- Heterozygous individuals have dwarf phenotype
Most of the population is homozygous recessive
- Recessive allele more prevalent in population
Dominant alleles causing lethal diseases much rarer than recessive alleles with lethal effects
– If it causes death before person can reproduce -> Allele not passed on
Timing of disease onset affects inheritance: Huntington’s disease
-Degenerative disease of nervous system
- Lethal dominant allele
- No obvious effect until individual is 35-40
Hemizygous
Male receiving gene from recessive X-linked trait
Sex-linked crosses: X-linked disorders
Color blindness
Duchenne muscular dystrophy
Hemophilia
X inactivation in female mammals
Most of one X chromosome in each female’s cells INACTIVATED
– Occurs during early embryonic development
– Barr body
Result
– Females and males, effectively, have one copy of X-
linked genes
Which X chromosome inactivates in each cell is random
– Females are mosaics of 2 cell types
Ex:
Tortoiseshell cats, inactivation of sweat glands
Recessively inherited disorders
Cystic fibrosis
Tay-Sachs disease
Sickle-cell anemia
