HOMOLOGOUS CHROMOSOME Pairs of matching chromosomes, with one being inherited from each parent.
ALLELES- alternative forms of the same gene. When 1 allele masks (hides) the effect of another, that allele is called DOMINANT and the hidden allele is called RECESSIVE. Dominant alleles are represented by a CAPITAL letter Recessive alleles are represented by a LOWERCASE letter A b C d e A B c d E Chromosome from MOM Chromosome from DAD ALLELES
What are Dominant Genes? Hereford: white face is dominant Horned is recessive to polled.
PHENOTYPE (what the organism LOOKS like) Examples of phenotypes: blue eyes, brown fur, striped fruit, yellow flowers GENOTYPE the genes present in the DNA of an organism 1 gene from MOM + 1 gene from DAD= 2 genes (2 letters) for offspring 3 possible GENOTYPES: "TT" – Homozygous Dominant "Tt" - Heterozygous "tt" – Homozygous Recessive
Genotype Symbol Genotype Vocabulary Phenotype TT homozygous DOMINANT or purebred tall tall Tt heterozygous or hybrid tall tt homozygous RECESSIVE or purebred short short
Gregor Mendel (1822-1884) Austrian monk Called the “Father of Genetics" for his study of the inheritance of 7 traits in pea plants.
Pollen transferred from white flower to stigma of purple flower all purple flowers result Mendel’s work ? self-pollinate Bred pea plants –cross-pollinate true breeding parents –raised seed & then observed traits –allowed offspring to self-pollinate & observed next generation
2 nd generation 3:1 75% purple-flower peas 25% white-flower peas Looking closer at Mendel’s work Parents 100% 1 st generation (hybrids) 100% purple-flower peas X true-breeding purple-flower peas true-breeding white-flower peas self-pollinate
Making crosses Can represent alleles as letters –flower color alleles P or p –true-breeding purple-flower peas PP –true-breeding white-flower peas pp PP x pp PpPp F1F1 P X purplewhite all purple
PP p Pp p all the F 1 (1 st filial generation) offspring are purple colored hybrids 100% purple offspring
Punnett squares Pp x Pp Pp male / sperm P p female / eggs PP 75% 25% 3:1 25% 50% 25% 1:2:1 % genotype % phenotype PPPpPp PpPppp PpPp PpPp 1 st generation (hybrids) Aaaaah, phenotype & genotype can have different ratios
Monohybrid Cross Example:Cross between two heterozygotes for brown eyes (Bb) BB = brown eyes Bb = brown eyes bb = blue eyes B b Bb Bb x Bb male gametes female gametes
Monohybrid Cross BB Bb bbB b Bb Bb x Bb 1/4 = BB - brown eyed 1/2 = Bb - brown eyed 1/4 = bb - blue eyed 1:2:1 genotype 3:1 phenotype
Dihybrid cross Other of Mendel’s experiments followed the inheritance of 2 different characters –seed color and seed shape –dihybrid crosses
Dihybrid cross true-breeding yellow, round peas true-breeding green, wrinkled peas x YYRRyyrr P 100% F 1 generation (hybrids) yellow, round peas Y = yellow R = round y = green r = wrinkled self-pollinate 9:3:3:1 9/16 yellow round peas 3/16 green round peas 3/16 yellow wrinkled peas 1/16 green wrinkled peas F 2 generation YyRr
Dihybrid Cross Example:cross between round and yellow heterozygous pea seeds. R= round r= wrinkled Y= yellow y= green RY Ry rY ry RY Ry rY ry RY Ry rY ry x RY Ry rY ry possible gametes produced RrYyRrYy RrYy x RrYy
Dihybrid cross YyRr YRYryR yr YR Yr yR yr YYRR x YYRrYyRRYyRr YYRrYYrrYyRrYyrr YyRRYyRryyRRyyRr YyRrYyrryyRryyrr 9/16 yellow round 3/16 green round 3/16 yellow wrinkled 1/16 green wrinkled YyRr YryRYR yr YyRr YRyr or
Incomplete Dominance Incomplete dominance is when both alleles are expressed and results in an intermediate phenotype. The hybrid is a blend of both alleles. When expressing incomplete dominant alleles, both alleles are represented by different capitalized letters.
Incomplete dominance Hybrids have “in-between” appearance –RR = red flowers –rr = white flowers –Rr = pink flowers make 50% less color RRRrrr RR WW RW
Incomplete dominance true-breeding red flowers true-breeding white flowers X P 100% 100% pink flowers 1 st generation (hybrids) self-pollinate 25% white 2 nd generation 25% red 1:2:1 50% pink
RR W W Homozygous Incomplete Dominance P (parental) generation
RW R R W W Incomplete Dominance F 1 (First Filial) generation 100% pink offspring
RW R W Heterozygous Incomplete Dominance F 1 x F 1
Incomplete dominance RW male / sperm R W female / eggs RR RWWW RW 25% 1:2:1 25% 50% 25% 1:2:1 % genotype % phenotype RR RW WW 25% 50% RW x RW
Co-dominance Some alleles are equally strong and neither are masked by the other. Alleles which are equally expressed are said to be "co-dominant". The hybrid is a mixture of both alleles.