Genetics

3.1 Genes

The Basics: Chromosomes Genes and Alleles

What is a gene? 

A gene is a heritable factor that consists of a length of DNA and influences a specific characteristic.

History Timeline: 

There are relatively few DNA molecules in a cell. Just 46 in a typical human cell, yet there are thousands of genes. We can, therefore, deduce that each gene consists of a much shorter length of DNA than a chromosome and that each chromosome carries many genes.  

What is the locus? 

The specific location of the gene on a chromosome

Where are the genes on the chromosome?

A gene occupies a specific position on one type of chromosome. When different varieties of plant or animal are crossed they show that genes are linked in groups and each group corresponds to one of the types of chromosome in a species. For example, there are four groups of linked genes and ten types of chromosome.

What are alleles? 

The various specific forms of a gene are alleles. 

What is a genome? 

The whole genetic information of an organism. 

 

3.2 Chromosomes

Bacterial chromosomes

Prokaryotes have one chromosome consisting of a circular DNA molecule. This chromosome contains all the genetic information the plant can have for life. The DNA in bacteria is not associated with proteins, so is sometimes described as naked. 

Plasmids are circular and naked and specific to prokaryotes and contain small extra DNA molecules. The DNA molecules contain few genes that may serve a use to the cell but are not needed for basic life processes. 

Haploid nuclei

Haploid nuclei have one chromosome of each pair. Haploid nuclei in humans are 23 chromosomes. 

Gametes are sex cells that fuse together during sexual reproduction. They have haploid nuclei since they are a haploid cell. Both egg and sperm have 23 chromosomes. 

Diploid Nuclei

Diploid nuclei have pairs of homologous chromosomes. Diploid nuclei in humans contain 46 chromosomes. 

Sex Characteristic/Gender

Sperm cell contains 22 autosomes and one copy of a sex chromosome, either an X or a Y. It is therefore the father's gamete that determines the sex of the offspring. 

xx = female

xy = male

 

 

 

3.3 Meiosis

Crossing over during prophase I. Homologous chromosomes come together and sections are exchanged between non-sister chromatids. This allows the mixing of alleles from the two parental chromosomes to form new, nearly limitless combinations in gametes. This occurs during gamete formation in both parents.

3.4 Inheritance

Definition: 

The inheritance of genes follows patterns ( genes are found in chromosomes in the nucleus of the cell) 

 

 

CHARACTERISTICS: ( where do they come from) 

 

 

Why haven’t we studied this more/ done meaningful results ? 

 

MENDEL’S LAWS OF INHERITANCE: Dominant alleles will overpower the recessive allele if they got one of each 



GREGOR MENDEL

 

Published 1866

Largely ignored at the time 

7 pairs of discrete characteristics 

Pea plants enable control of pollination 

Large volumes of QUANTITATIVE results 





Segregation of Alleles:  1) ANAPHASE 



PEA PLANTS 

TALL (PHENOTYPE)

SHORT

(PHENOTYPE) 

 

PARENTAL GENERATION

TT ( GENOTYPE)



HOMOZYGOUS DOMINANT

tt (GENOTYPE) 



HOMOZYGOUS RECESSIVE 

 



Punnett square= what can be produced 

Phenotype = physical manifestation of the genotype ( WHAT THEY LOOK LIKE) 

Genotype= the genetic makeup that each parents has Genetics 

Heterozygous = different alleles ( from different parents)

Test cross = Any time that we do a test cross we have to include a homozygous recessive individual ( WHY ? Because we are trying to figure out if this dominant characteristic is homozygous or heterozygous) 



Heterozygous alleles 75% Tall

T

t

T

TT

Tt

t

Tt

tt



OPTIONS: MONOHYBRID CROSS 

GENOTYPE

PHENOTYPE

  • 25 % TT
  • 75% Tall
  • 25% tt

25% Short 

  • 50% Tt
 






Co dominance: CRor Cw

 

Codominance of Red flower and White flower

CR

CW

CR

   

CW

   

 

Eg: ABO Blood Groups 

Type O = ii 

Type AB = IAIB

What if the allele that codes for blood type is given to the offspring from grandparents. 2

 

When using test crosses we must use a homozygous recessive allele. 

 

Type O blood cannot receive type A or type B= Sending white blood cells to repel blood that doesn’t belong in the body 

 

TYPE O= Universal donor ( anyone can receive it) ( 0+ any positive blood type can receive it) (o- anyone - and + blood types)

 

Type  AB =  Universal recipient (any blood type can be absorbed.)

 

RH factor: Additional recognition factor on the surface it can be positive or negative. 




SEX LINKAGE 



Female= XX 

Male= XY 

 

XX

XX

XY

XY

 

HEMOPHILIA= Recessive characteristic present only on the X chromosome. Causes a rare disease that prompts the cells to blood clot and die. 

 

 

XH

Xh

Xh

XHXh

XhXh

Y

XH

XhY 

 

Why is it more common for men  to be hemophiliac than women ? 

Cus Aj Trcy to the hyper man set. Postman pat and jess doing ket 

Editors

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