Mendel and the Gene Idea - GitHub Pages complexities of inheritance •Most genetic traits are not as simple as the ones that Mendel studied. •Incomplete dominance, codominance,
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Mendel's model1.Alternative versions (alleles) of hereditary factors (genes) account for variability in inherited traits 2.An organism inherits two alleles, one from each parent3.If two alleles differ, then the trait is determined by the dominant allele; the recessive allele has no effect on appearance4.the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes (this is now known as the law of segregation)5.Alleles for separate genes are passed independently of each other (this is now known as the law of independent assortment – more on this later)
Figure 14.5-3
P Generation
Appearance:Genetic makeup:Gametes:
Purple flowers White flowersPP ppP p
F1 Generation
Appearance:Genetic makeup:Gametes:
Purple flowersPp
P p1/2 1/2
F2 GenerationSperm from F1 (Pp) plant
P
P
p
p
Eggs from F1 (Pp) plant
PP
ppPp
Pp
3 : 1
• The possible combinations of sperm and egg can be shown using a Punnett square, a diagram for predicting the results of a genetic cross between individuals of known genetic makeup
• A capital letter denotes the dominant allele while lowercase denotes a recessive allele
Useful Genetic Vocabulary
• An organism with two identical alleles for a character is said to be homozygous for the gene controlling that character
• An organism that has two different alleles for a gene is said to be heterozygous for the gene controlling that character
• We distinguish between an organism’s phenotype, or physical appearance, and its genotype, or genetic makeup
Concept 14.3: Inheritance patterns are often more complex than predicted by simple Mendelian genetics• The relationship between genotype and phenotype is
rarely as simple as in the pea plant characters Mendel studied
• Many heritable characters are not determined by only one gene with two alleles
• However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance
• For example, ABO blood typing is determined by three alleles and is an example of codominance
Additional complexities of inheritance• Most genetic traits are not as simple as the ones
that Mendel studied. • Incomplete dominance, codominance, multiple
alleles, pleiotropy, epistasis, and polygenic inheritance are described on the remaining slides
Figure 14.UN03
Complete dominanceof one allele
Relationship amongalleles of a single gene Description Example
Incomplete dominanceof either allele
Codominance
Multiple alleles
Pleiotropy
Heterozygous phenotypesame as that of homo-zygous dominant
Heterozygous phenotypeintermediate betweenthe two homozygousphenotypes
Both phenotypesexpressed inheterozygotesIn the whole population,some genes have morethan two allelesOne gene is able to affectmultiple phenotypiccharacters
ABO blood group alleles
Sickle-cell disease
PP Pp
CRCR CRCW CWCW
IAIB
IA, IB, i
Figure 14.UN04
Epistasis
Polygenic inheritance
Relationship amongtwo or more genes Description Example
The phenotypicexpression of onegene affects thatof another
A single phenotypiccharacter is affectedby two or more genes