Term 3 - D3.2 Inheritance

D3.2.1 Haploid (3)

• A haploid cell only has one single set of chromosomes

• Written as n

• In humans, egg and sperm cells are haploid and contain 23 chromosomes. When they join together, they make a full diploid cell with 46 chromosomes

D.3.2.1 Diploid (3)

• A diploid cell or organism has two complete sets of chromosomes, with one set coming from each biological parent

• Written as 2n

• In humans, this equals 46 total chromosomes (23 pairs)

D.3.2.1 Gametes (4)

• Gametes are sex cells used in sexual reproduction

• Male gametes are sperm, and female gametes are eggs

• Produced through meiosis

• Each gamete is haploid --> fuse together to create a diploid zygote

D3.2.1 Zygote (3)

• A zygote is the single cell formed when a sperm cell and an egg cell join together during fertilization

• It carries a complete set of DNA from both parents and marks the very first developmental stage of a new living organism

• Each zygote inherits two copies (genetic inheritance) - one from mom and one from dad (alleles)

D3.2.2 Generations (3)

• P generation (parent generation) - the original parents in a genetic cross

• F1 generation (first filial generation) - the offspring of the P generation

• F2 generation (second filial generation) - the offspring of the two individuals from the F1 generations

D.3.2.2 Genes (1)

Genes are sections of DNA that code for a specific protein, and determine a trait

D.3.2.3 Alleles (4)

• Alleles are alternative forms of a gene

• Produced by mutations to a gene

• Organisms which have cells with diploid nuclei will have two copies of each gene

• Diploid organisms like us contain two alleles for each gene - one from each parent

D.3.2.3 Genotypes (3)

• Homozygous: individuals have two identical copies of an allele for a gene

• Heterozygous: individuals have two different copies of an allele for a gene

• Genotype: is the combination of alleles inherited by an organism

D.3.2.4 Phenotypes (1)

Phenotypes are the observable traits of an organism determined by the genotype interacting with the environment

D.3.2.4 Traits determined by genotype only (3)

• Blood type

• Genetic diseases

• Eye colour

D3.2.4 Traits determined by genotype interacting with the environment (2)

• Height

• Athletic performance

D.3.2.4 Traits determined solely by environment (1)

Language spoken and accent

D3.2.5 Dominant Alleles (3)

• Dominant alleles are alleles that have the same effect on the phenotype in homozygous and heterozygous individuals

• The dominant allele is always expressed in the phenotype

• Dominant alleles mask the effect of recessive alleles --> an organism will express the dominant trait if they inherit at least one copy of it

D3.2.5 Recessive Alleles (3)

• Recessive alleles that have an effect on the phenotype of a homozygous individual with two recessive alleles of the gene

• Recessive alleles are only expressed when an individual has two recessive alleles for a trait

• Recessive alleles only dictate the physical phenotype when an individual inherits two copies (one from each parent)

D3.2.5 Monohybrid Cross (3)

• E.g. The presence of freckles is a trait controlled by a dominant allele in humans. If two parents are heterozygous for the trait, determine the probability of them having a child with freckles

• Phenotype: Freckles x Freckles

• Genotype: Ff x Ff

D3.2.6 Phenotypic Plasticity (2)

• Phenotypic plasticity refers to reversible changes in phenotype suited in the environment due to changing patterns in gene expression

• Is not due to changes in genotype, and the changes in traits may be reversible during the lifetime of an individuals

D3.2.6 Phenotypic Plasticity - face freckle example (2)

• Face freckle gene (FF) --> tend to have pale skin, freckles, red hair

• Example of phenotypic plasticity: Irish girl has freckles but moves to Australia --> more sun more UV --> pale skin becomes tan but freckles stay (genes stay the same but cells change)

D3.2.7 Most genetic diseases are caused by the recessive allele (2)

• People who are homozygous (dd) recessive will display symptoms of the diseases

• Those who are heterozygous (Dd) will be carriers of the disease - no symptoms

D3.2.7 Phenylketonuria (PKU) (4)

• Phenylketonuria (PKU) is a genetic disease where an individual does not have a functioning gene to make the enzyme that converts the amino acid phenylalanine to tyrosine

• Caused by a recessive allele

• People with PKU are homozygous recessive for the disease

• High concentrations of phenylalanine build up in the brain, causing intellectual disability, delayed development, behavioural, emotional, and social problems

D3.2.8 Gene Pools (4)

• A gene pool is all of the different genes and alleles present within an interbreeding population of a species

• There can be many different alleles of a gene in any gene pool

• Many of the alleles are the result of base substitutions of the gene forming new variations of the gene known as single-nucleotide polymorphisms

• Even though there may be many alleles of a gene in a population, an individual will only have two copies of the gene, inheriting one allele from each parent

D3.2.9 ABO Blood Groups (1)

ABO blood groups are based on the presence of antigen proteins on the plasma membrane of red blood cells. ABO blood types is an example of a gene with multiple alleles

D3.2.9 ABO Blood Group Alleles (2 + 3)

• Multiple alleles occur when there are more than two alleles for a gene

• There are three common alleles (I^A, I^B and i) for human blood type:

  • I^A which is dominant to the allele i, but codominant to the allele I^B

  • I^B which is dominant to the allele i, but codominant to the allele I^A

  • i which is a recessive allele to the alleles I^A and I^B

• i is the recessive allele

• IA and IB are dominant over i

• IA and IB are codominant

D3.2.9 Example of Codominance - ABO Blood Groups

Phenotype/Blood Type (4 possible phenotypes)	Genotype (6 possible genotypes)	Antigens on RBCs	Antibodies in plasma	Can accept blood from
A	IAIA or IAi	A	B	A, O
B	IBIB or IBi	B	A	B, O
AB (universal recipient)	IAIB	AB	No antibodies	A, B, AB, O
O (universal donor)	ii	No antigens	AB	O

D3.2.10 Codominance (4)

• Codominance occurs when heterozygotes have a dual phenotype

• Both alleles are dominant, and both are expressed in the phenotype

• The inheritance of blood type AB (I^AI^B) is an example of codominance

• The protein coded by both versions of the allele is expressed as an antigen on the red blood cell for a person with an AB (I^AI^B) blood group

• ABO blood group - both alleles are expressed in the phenotype

• E.g. blood group AB - will express both A and B antigens on the surface

D3.2.10 Incomplete Dominance (3)

• Incomplete dominance occurs when heterozygotes have an intermediate phenotype to homozygous individuals

• Neither allele is dominant

• The inheritance of flower colour for the four o’clock flower (Mirabilis jalapa) is an example of incomplete dominance

D3.2.10 Four o’clock Flower (5)

• There are three phenotypes for flower colour in four o'clock plants

• Red, when the plant has two red alleles for flower colour

• White, when the plant has two white alleles for flower colour

• Pink, when the plant is heterozygous for flower colour

• Pink is the intermediate phenotype between the red and white phenotypes

D3.2.11 Sex Chromosomes and Autosomes (5)

• X and Y chromosomes are the sex chromosomes and determine biological sex

• The X chromosome is longer than the Y chromosome and contains many more genes

• The Y chromosome contains an SRY gene. If a human embryo has an SRY gene, it will develop male-typical characteristics

• The X chromosome does not contain an SRY gene. An embryo with two X chromosomes does not have an SRY gene, and develops female-typical characteristics

• All other chromosomes apart from the sex chromosomes, are autosomes

D3.2.11 Sex Determination in Humans (5)

• Females = XX; males = XY

• All female gametes and eggs will have one X chromosome

• The male gametes and sperm will have either the X or the Y chromosome

• Eggs fertilised by a sperm with an x chromosome will develop female-typical characteristics (XX)

• Eggs fertilised by a sperm with a Y chromosome will develop male-typical characteristics

D3.2.11 Example of Sex Linkage using Drosophila (fruit flies)

• Drosophila are fruit flies

• Gene that determines eye colour in drosophila is found on the X chromosome

 

Cross a red eye dad and a white eye mom

	XR	Y
Xr	XRXr	XrY
Xr	XRXr	XrY

Y: Y chromosome in males

XR: red eye allele (dominant phenotype)

Xr: white eye allele

 

Genotype: 50% XRXr : 50% XrY

Phenotype: 50% red eye female : 50% white eye male

 

1. What is the percentage of females that will display a red eye phenotype?

100% (talking abt a specific gender, not the whole offspring) - DON’T GET TRICKED BY THIS !!

D3.2.12 Sex-Linked Traits (4)

• The sex chromosomes are homologous chromosomes, and should have the same genes

• Sex-linked traits are traits where the allele is present on only one sex chromosome (usually X)

• The Y chromosome is much shorter than the X chromosome. Therefore there are many genes that are on the X chromosome, but not on the Y chromosome

• Females will get 2 copies of these sex linked genes, but males will only get one copy

D3.2.12 Haemophilia (4)

• Haemophilia is a sex-linked disease caused by a mutation to a gene for making the blood clotting factor XIII

• The gene is on the X chromosome, but not on the Y chromosome

• Males carry one copy of the gene on their X chromosome. There is no allele on the Y chromosome

• Females carry two copies of the gene, one allele on each X chromosome

D3.2.13 Pedigree Charts (1)

A pedigree chart is a diagram that shows the occurrence and appearance of phenotypes of a particular gene or organism and its ancestors from one generation to the next

D3.2.14 Polygenic Inheritance (2)

• Polygenic inheritance is a single characteristic controlled by multiple genes

• Human skin colour is an example of a polygenic inheritance

D3.2.14 Inheritance of Skin Colour humans (6)

• Skin colour depends on the amount and type of melanin represent in the skin

• Melanin is a brown protein pigment

• There are at least four genes which control the amount of melanin present in the skin

• Each gene has multiple alleles, which may promote or inhibit melanin production

• The alleles may be codominant

• The inheritance of skin colour in a human is a polygenic trait

D3.2.14 Discrete Inheritance (6)

• Discrete variation results from one gene being responsible for a trait

• All individuals fit into a number of non-overlapping phenotype - phenotypes fall into distinct categories

• Determined by a single gene (monogenic)

• Not affected by environment

• Frequency tables, pie charts, bar charts

• Examples: ABO blood groups, haemophilia, eye colour, whether you have a disease or not

D3.2.14 Continuous Inheritance (

• Continuous variation results from polygenic inheritance, where multiple genes determine the phenotype (polygenic)

• Affected by environmental factors (nature and nurture)

• Produces a range of overlapping phenotypes between two extremes

• Phenotype are measured across a range, usually follows a normal distribution (bell curve)

• Box and whisker

• Examples: Human skin colour, height, weight

D3.2.15 Box-and-Whisker Plot (7)

• IQR = Q3 - Q1

• Q3: upper quartile, 75%

• Q1: lower quartile, 25%

• Q2: median

• Minimum = Q1 - IQR*1.5

• Maximum = Q3 + IQR*1.5

• Anything that falls out of the min and max are outliers