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: 

• 19th Century: experiments were done which showed that there were factors in living organisms that influenced specific characteristics and scientists showed these factors were heritable. Experiments such as Gregor Mendel's Pea plant experiment proved this but other scientists only took notice after he died. 
• Early 20th Century: Research became more intense. The word gene was invented for the heritable factors.
• Mid 20th Century: There was strong evidence that genes were made of DNA.

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.

• Crossing over between homologues in prophase I
• Random orientation of tetrads in metaphase I
• The fusion of gametes from two individuals

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) 

• Environment ( skin colour/ height - gravity) 
• Parents 

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

• No one wants to volunteer to mix genes with someone else 
• Lots of people need to be involved

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 

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 

• Homozygous dominant or Homozygous recessive 
• TT = Homozygous Dominant
• tt = Homozygous Recessive 

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) 

OPTIONS: MONOHYBRID CROSS 

Co dominance: CRor Cw

• Both top letters have to be capital 
• When the outcome is a mixture of both alleles ( White+ red= Pink) 

Eg: ABO Blood Groups 

• Three alleles IA,IBand i for glycoprotein on cells
• Possible phenotypes = genotype needed

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 

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

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

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