CHAPTER 2:

GENETIC INHERITANCE

Concept review…

Genetic inheritanceTerms & concepts

Mendel’s experiment

Pisum sativum – garden pea

Monohybrid crosses

Dihybrid crosses

Law of segregatio

n

Law of independe

nt assortment

Deviation from Mendel’s

Law

Codominance

Incomplete dominance

Multiple allele

Polygene

Lethal allele

Linked gene

Genetic Genetic mappingmapping

Pedigree Pedigree analysisanalysis

Important terms…Important terms…

alleleallelegenesgenes

locuslocus

genotypegenotype

phenotypephenotype

dominantdominant

recessiverecessive

homozygoushomozygousheterozygousheterozygous

true-breed

OBJECTIVE

• Define terminologies used in genetic inheritance.

• Describe the characteristics of Mendel’s pea plants.

Terms used in genetic inheritance

• Alleles:

all the different molecular forms of the same genes

• 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.

Locus• 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.

GenotypeParticular genes and individual carries / the genes that an organism inherits from its

mother and father.

PhenotypeAn individual’s observable traits/ the protein used by these genes that determine the

organism’s physical characteristic.

Eg: 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-breedTrue-breed @ pure-breed

Organisms that are homozygous for any Organisms that are homozygous for any given genotype and therefore pass it on to given genotype and therefore pass it on to all their progeny in a cross with a similar all their progeny in a cross with a similar homozygotehomozygote

• P : parental generation• F1 : first-generation offspring• F2 : second-generation

offspring

TYPES OF CROSSESTest crossA cross between a recessive homozygous and

an organism of dominant phenotype, but unknown genotype.– 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.

Self cross– Male and female from the same

plants /generation

Back cross– A mating between individuals of the

parental generation (P) and the first generation (F1) in order to identify hidden recessive alleles.

Reciprocal crossA 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 : XbXb x XBY color blind normal

G : Xb Xb XB Y

F1 : XBXb XBXb XbY XbY

EARLIER CROSS

B – normalb – color blind

normal female Color blind male

Phenotype ratio : 1 normal : 1 color blind

P : XBXB x XbY normal color blind

G : XB XB Xb Y

F1 : XBXb XBXb XBY XBY

RECIPROCAL CROSS

Female carrier Normal male

Phenotypic ratio : All normal

Mendel’s Experiment

Gregor Mendel studied the inheritance ofcharacteristics 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

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Mendel’s Experimental Result

Character Dominant vs F2 Ratio recessive

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.

• F2 generation showed close to 3:1 ratio of characteristics.