IGCSE Biology - Inheritance

DNA, RNA, genes, chromosomes, phenotype, etc.

Inheritance

the reception of genetic qualities by transmission from parent to offspring.

DNA

Deoxyribonucleic acid is the molecule that carries genetic information for the development and functioning of an organism. It is made of two linked strands that wind around each other to resemble a twisted ladder — a shape known as a double helix.

Genes

A gene is the basic physical and functional unit of heredity. Genes are made up of DNA. Some genes act as instructions to make molecules called proteins.

Allele

One of two or more versions of a genetic sequence at a particular region on a chromosome. An individual inherits two of them for each gene, one from each parent.

Who came up with the double helix DNA?

Rosalind Franklin, James Watson & Francis Crick

Dominant & recessive genes

An allele of a gene is said to be dominant when it effectively overrules the other (recessive) allele. Eye colour and blood groups are both examples of dominant/recessive gene relationships.

Dominant & recessive genes capitalisation

Dominant = gets capital letters

Recessive = gets lowercase letters

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Example:

Dominant = Aa/AA

Recessive = aa

Blue eyes

Recessive trait

Widow’s peak

Dominant trait

Fraternal twins

also called dizygotic twins, result from the fertilisation of two separate eggs with two different sperm during the same pregnancy.

Identical twins

also called monozygotic twins, result from the fertilisation of a single egg by a single sperm, with the fertilised egg then splitting into two.

RNA

Ribonucleic acid is a nucleic acid present in all living cells that has structural similarities to DNA. Unlike DNA, however, RNA is most often single-stranded. An ribonucleic acide molecule has a backbone made of alternating phosphate groups and the sugar ribose, rather than the deoxyribose found in DNA.

Chromosomes

A structure found inside the nucleus of a cell. A chromosome is made up of proteins and DNA organized into genes. Each cell normally contains 23 pairs of chromosomes.

Female sex chromosomes

XX

Male sex chromosomes

XY

Functions of chromosomes

to carry the basic genetic material – DNA. DNA provides genetic information for various cellular functions. These functions are essential for the growth, survival, and reproduction of the organisms. Histones and other proteins cover the Chromosomes.

Phenotype

an observable type of an organism, and it can refer to anything from a common trait, such as height or hair color, to presence or absence of a disease.

Heterozygous

refers to having different alleles for a particular trait. If the two versions are different, you have a heterozygous genotype for that gene. The relationship between the two alleles affects which traits are expressed.

Homozygous

refers to having inherited the same versions (alleles) of a genomic marker from each biological parent. Thus, an individual who is homozygous for a genomic marker has two identical versions of that marker.

Dominant genes

• Produce the complete polypeptide

• Mask the effect of recessive genes

Polypeptide

A substance that contains many amino acids (the molecules that join together to form proteins).

Recessive genes

• Produce an incomplete polypeptide

• Unable to mask the effect of the dominant genes

Law of independent assortment

describes how different genes independently separate from one another when reproductive cells develop. Independent assortment of genes and their corresponding traits was first observed by Gregor Mendel in 1865 during his studies of genetics in pea plants.

Gregor Mendel

through his work on pea plants, discovered the fundamental laws of inheritance

Punnett square

a table in which all of the possible outcomes for a genetic cross between two individuals with known genotypes are given. the table is divided into four quadrants.

Some exceptions in Mendel’s principle

A situation in which neither allele is dominant

• When both alleles are present, a “new” phenotype appears that is a blend of each allele

• The alleles will be represented by capital letters

AKA Incomplete dominance

ACGT

ACGT is an acronym for the four types of bases found in a DNA molecule: adenine (A), cytosine (C), guanine (G), and thymine (T). A DNA molecule consists of two strands wound around each other, with each strand held together by bonds between the bases.

Adenine pairs with

Thymine

Cytosine pairs with

Guanine

Rosalind Franklin

She was a British chemist and X-ray crystallographer whose work was central to the understanding of the molecular structures of DNA, RNA, viruses, coal, and graphite.

James Watson

an American molecular biologist, geneticist, and zoologist. In 1953, he co-authored an academic paper proposing the double helix structure of the DNA molecule

Francis Crick

was an English molecular biologist, biophysicist, and neuroscientist. He, James Watson, Rosalind Franklin, and Maurice Wilkins played crucial roles in deciphering the helical structure of the DNA molecule.

Function of DNA

to control cell function by controlling the production of proteins i.e. enzymes, antibodies, cell receptors, etc.

Protein manufacture

a gene is a particular length of a DNA molecule containing a specific sequence of bases. This base sequence codes for which amino acids should be joined in what order, to build a particular protein. 

Diploid cells

has two complete sets of chromosomes. Most cells in humans are diploid, comprising 23 chromosome pairs, so 46 chromosomes in total.

Haploid cells

describes a cell that contains a single set of chromosomes. The term can also refer to the number of chromosomes in egg or sperm cells, which are also called gametes.

Gametes

 a reproductive cell of an animal or plant. In animals, female gametes are called ova or egg cells, and male gametes are called sperm. Ova and sperm are haploid cells, with each cell carrying only one copy of each chromosome. During fertilisation, a sperm and ovum unite to form a new diploid organism.

Diplod & Haploid

• Diploid cells have 23 pairs of chromosomes

• Haploid cells have a single set of 23 chromosomes

Mitosis

a type of cell division that results in two daughter cells each having the same number and kind of chromosomes as the parent nucleus, typical of ordinary tissue growth.

Mitosis functions

• Growth

• Repair

• Cell replacement

Meiosis

a type of cell division that results in four daughter cells each with half the number of chromosomes of the parent cell, as in the production of gametes and plant spores.

Genotype

the genetic constitution of an individual organism.

Monohybrid cross

 a cross between two organisms with different variations at one genetic locus of interest. The character being studied in a monohybrid cross are governed by two or multiple variations for a single location of a gene.

Co-dominance

when a pair of alleles are neither dominant to one another. Both alleles can therefore impact the phenotype. 

Sex-linked characteristics

Sex linked characteristics are characteristics in which the gene responsible is located on the sex chromosome, and therefore making it more common in one sex than the other. 

Colour blindness

The X chromosome (Xc) contains the faulty gene that causes color deficiencies.

Because males only have one X chromosome, this abnormal gene will always be expressed, resulting in color deficiency in a male (with XY chromosomes).

On the other hand, females may carry the gene for color blindness yet still have normal color vision. Females have two X chromosomes (XX), which accounts for this.

As long as a female has one normal X chromosome, she will have good color vision because the normal allele for color vision predominates over the defective allele.