Biology Chapter 18

What is inheritance?

Inheritance refers to the transmission of traits from one organism to another

What is monohybrid inheritance?

Monohybrid inheritance refers to the inheritance of one characteristic that has 2 contrasting forms(e.g. tall and short gene)

What are pure-bred plants?

Plants that are self-pollinated, and produce offspring resembling the parent

What do you call the 1st and 2nd generation of offspring?

F₁ generation and F₂ generation

How did the F₂ generation come about?

When the F₁ generation self-fertilised

In an experiment, the smaller the sample sizes, …

the less accurate the ratios are

What is the nature of fertilisation?

Random

What is a gene?

A gene is a unit of inheritance. It is a sequence of DNA nucleotides that controls the formation of a single polypeptide

What are alleles?

Alleles are different forms of the same gene and occupy the same relative position on a pair of homologous chromosomes

If a person has 2 different alleles, what do you call the allele that expresses itself?

The dominant allele

If a person has 2 different alleles, what do you call the allele that does not express itself?

The recessive allele

A capital letter represents the dominant allele, while a small letter represents …

the recessive allele

What is a genotype

The combination of alleles for a particular gene

What is a phenotype?

The expressed trait or outward appearance

When both the alleles in a particular gene are identical, the genotype is referred as ….

homozygous

What are the 2 conditions for an organism to have identical alleles?

It is either pure bred or homozygous

When the alleles are different for a particular gene, the genotype is referred to as …

heterozygous

What are the 3 possible descriptions of a genotype?

Homozygous recessive (dd)

Homozygous dominant (DD)

Heterozygous (Dd)

A dominant allele expresses itself in both the homozygous dominant and the heterozygous condition. (T/F)

True

When does the recessive allele express itself?

In the absence of the dominant allele, so it can only express itself in the homozygous recessive condition

What is the purpose of a genetic diagram?

It explains how alleles are passed on to an offspring

What are the 7 steps to a genetic diagram?

1. Legend

2. Parental phenotypes

3. Parental genotypes

4. Gametes

5. Fertilisation of gametes (Punnett sqaure)

6. F₁ genotype

7. F₁ phenotype

8. F₁ phenotype ratio

If an organism is heterozygous, …

50% of the offspring will show the dominant phenotype, while the other 50% will show the recessive phenotype

In a heterozygous condition, if both alleles are expressed to produce a phenotype that is intermediate between the dominant and the recessive phenotypes, the alleles are said to be …

codominant

What are the genotypes of different blood groups?

Blood group A = I^AI^A or I^AI^O

Blood group B = I^BI^B or I^BI^O

Blood group AB = I^AI^B

Blood group O = I^OI^O

I^A and I^B are codominant with each other. (T/F)

True

I^O is recessive to I^A and I^B (T/F)

True

How many pairs of chromosomes do humans have?

23 pairs of chromosomes: 22 pairs of autosomes( non-sex chromosomes), 1 pair of sex chromosomes

What are the sex chromosomes in males?

Sex chromosomes X and Y (Genotype: XY)

What are the sex chromosomes in females?

Sex chromosomes X only (Genotype: XX)

In females, the eggs produced will all contain …

22 autosomes and one X chromosome

In males, the sperm will contain …

22 autosomes and either one X or one Y chromosome

What ensures equal chances of having a baby boy or baby girl?

Random fertilisation, which has a 1:1 ratio

What is variation?

Variation refers to the differences in traits between individuals of the same species

What are the 2 types of variations?

Discontinuous variation (very few phenotypes)

Continuous variation (multiple/range of phenotypes)

Characteristics of discontinuous variation

Distinct, clear-cut, easily distinguishable phenotypes with no intermediates

Controlled by one of a few phenotypes

Genes do not have an additive effect

Relatively unaffected by environmental conditions

Characteristics of continuous variation

Range of phenotypes

Controlled by many genes

Genes show an additive effect

Greatly affected by environmental conditions

What are examples of discontinuous variation?

Ability to roll tongue

Blood type

Iris colour

Earlobe type

Eyelid type

What are the examples of continuous variation?

Height

Weight

IQ

Skin colour

Finger/Toe length

Shoe size

Size of eye

What are sources of genetic variation?

Meiosis
Random fertilisation

Mutation

What is meiosis?

It produces genetically dissimilar gametes with different combinations of the parental chromosomes

What is random fertilisation?

A random sperm from the millions of sperms released fuses with the egg to form a unique zygote

What is mutation?

It is a change in the sequence of a gene or in the chromosome number

A change in the sequence of a gene can result in …

a change in the amino acid sequence of the polypeptide formed

A mutation that occurs during gamete production can result in …

genetic changes inherited by the offspring

What are 2 examples of gene mutation?

Albinism

Sickle cell anaemia

How do people inherit albinism?

People who are homozygous recessive for the albinism allele are albinos

What are the characteristics of a person who is an albino?

Absence of pigments in the skin, hair and eyes causes albinos to have reddish-white skin and white hair

Absence of pigment in the iris of albinos causes the eyes to appear red

Albinos are sensitive to sunlight and get sunburnt easily

How do people inherit sickle cell anaemia?

People who are homozygous recessive for the haemoglobin S (HbS) protein in red blood cells suffer from sickle cell anaemia

What are the symptoms of people with sickle cell anaemia?

Have sickle-shaped red blood cells that have reduced oxygen transport efficiency

Sickle-shaped cells are fragile, sticky and can get in narrow blood cells, slowing or cutting off oxygen transport

What is chromosome mutation?

A change in the number of chromosomes, deviating from the original 46 chromosomes in a normal human cell

An individual with 47 chromosomes, with an extra copy of chromosome 21, suffers from…

Down syndrome

How does Down syndrome occur?

Can occur in an offspring where both parents are normal

The older the mother, …

the higher the chances of Down syndrome in the offspring

The mutation can also occur in males, resulting in the formation of a sperm with an extra copy of chromosome 21. (T/F)

True

What causes the rate of spontaneous mutation to increase?

Presence of mutagens

What are examples of mutagens?

Ionising radiation - UV light, alpha, beta and gamma radiation, X-rays

Chemical mutagens - tar and formaldehyde, lysergic acid diethylamide(LSD)

What are some environmental factors that affect why individuals produce more offspring than their environment may be able to support?

Competition for food and space

Diseases

Predation

Random and spontaneous selection gives rise to …

favourable genes in organisms

What is natural selection?

Natural selection refers to a process in which organisms with desirable genes and characteristics are “selected” or “favoured” to survive and pass on their genes to their offspring

What does mutation provide?

It provides new genes or alleles for natural selection to operate on

After hundreds of years, …

more beneficial qualities may accumulate in a species

A new breed may be …

better adapted to the new environment

An organism might change so much that …

they become a new species, which is different from the original species

What is evolution?

Evolution is the gradual change in the inheritable characteristics of a population over time

How does natural selection drive evolution?

Environmental pressures → Competition for survival → Mutation → Variation → Natural selection: survival of the fittest → Reproduction and passing down of favourable genes → Evolution

How did dark peppered moths undergo natural selection?

Burning of coal in factories due to industrialisation covered tree trunks with soot, allowing dark peppered moths to be able to camouflage better. This led to mutation of 2 types of moths, white and dark peppered. Black peppered moths survive, reproduce and pass on their traits, causing the population to increase, while white peppered moths are preyed upon.

How did antibiotic-resistant bacteria undergo natural selection?

The indiscriminate use or abuse of antibiotics led to bacteria with mutations that made them resistant to certain types of antibiotics. Bacteria with the mutation could survive, reproduce and pass on their favourable genes, causing the population to increase. Bacteria without this mutation will die. This led to the evolution of antibiotic-resistant bacteria.

How did Darwin’s finches undergo natural selection?

Mainland ancestral finches migrated to different islands and reproduced rapidly, causing competition for food to occur. This led to mutation, where finches with different types of beaks occurred. Finches with beaks better suited to a particular diet on the islands survived, reproduced and passed on their traits, causing the population to increase. Finches with beaks not suited for the diet on the island could not survive. This led to the evolution of more than 6 different species, which were adapted to a particular food source.