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Concept review… Genetic inheritance Terms & concepts Mendel’s experiment Pisum sativum – garden pea Monohybrid crosses Dihybrid crosses Law of segregation Law of independent assortment Deviation from Mendel’s Law
Genetic mapping Pedigree analysis
Important terms… allele genes locus genotype phenotype dominant recessive homozygous heterozygous true-breed
Describe the characteristics of Mendel’s pea plants.
Terms used in genetic inheritance
all the different molecular forms of the same genes
units of information about specific traits and they are passed from parents to offspring/ small section of DNA that codes for a particular protein. Cells with a diploid chromosome number (2n) have pairs of genes, on pairs of homologous chromosomes.
Each gene has a specific location locus on a chromosome .
P Q p q
There are two loci , one is for flower color and the other is for stem length
Two terms help keep the distinction clear between genes and the traits they specify.
Particular genes and individual carries /
the genes that an organism inherits from its mother and father.
An individual’s observable traits/ the protein
used by these genes that determine the organism’s
Phenotype : tall
Genotype : TT @ Tt
An allele is said to be dominant when its effect on a trait masks that of any recessive allele paired with it. We used capital letters for dominant alleles and lowercase letters for the recessive one. Example, A and a.
So, the homozygous dominant has a pair of dominant alleles (AA), homozygous recessive has a pair of recessive alleles (aa) and heterozygous has a pair of non-identical allele (Aa).
True-breed @ pure-breed
Organisms that are homozygous for any given genotype and therefore pass it on to all their progeny in a cross with a similar homozygote
P : parental generation
F1 : first-generation offspring
F2 : second-generation offspring
TYPES OF CROSSES
A cross between a recessive homozygous and an
organism of dominant phenotype , but unknown
If an organism displays a dominant characteristic, it may possess 2 dominant alleles (homologous) or a dominant and recessive allele for that characteristic (heterozygous)
To find out which in the case, the organism is crossed with one displaying the recessive characteristic.
If all the offspring show the dominant characteristic then the organism is homozygous, but if half show the recessive characteristic, then the organism is heterozygous.
Male and female from the same plants /generation
A mating between individuals of the parental generation (P) and the first generation (F1) in order to identify hidden recessive alleles.
A cross reversing the roles of males and females to confirm the results obtained from an earlier cross.
For example, if a pollen (male) from tall plants in transferred to the stigma (female) of dwarf plants in one cross, the reciprocal cross would use the pollen of dwarf plants to pollinate the stigmas of tall plants.
P : X b X b x X B Y color blind normal G : X b X b X B Y F1 : X B X b X B X b X b Y X b Y EARLIER CROSS B – normal b – color blind normal female Color blind male Phenotype ratio : 1 normal : 1 color blind
P : X B X B x X b Y normal color blind G : X B X B X b Y F1 : X B X b X B X b X B Y X B Y RECIPROCAL CROSS Female carrier Normal male Phenotypic ratio : All normal
Gregor Mendel studied the inheritance of
characteristics in garden peas ( Pisum sativum ).
He chose peas because :
they were easy to grow
they had a short life cycle
their pollination could be controlled
they have easily observable characteristics .
He studied 7 characteristics, each of which
has two contrasting alternatives.
seed shape : round / wrinkled
seed color : yellow / green
pod shape : inflated / constricted
pod color : yellow / green
flower color : purple / white
flower position : axial / terminal
plant height : tall / dwarf
Mendel’s Experimental Result
Character Dominant vs F 2 Ratio
Seed shape R x r 5474 : 1850 2.96 :1
Seed color Y x y 6022 : 2001 3.01 :1
Flower color P x p 705:224 3.15 : 1
Pod shape I x i 882:299 2.95 : 1
Pod color G x g 428:152 2.82 : 1
Flower position A x a 651:207 3.14 : 1
Plant height T x t 787:277 2.84 : 1
It shows 7 characters studied in garden pea crosses and the data obtained.
F 2 generation showed close to 3:1 ratio of characteristics.