D3.2 INHERITENCE

Discuss the production of haploid gametes and diploid zygotes

Discuss the production of haploid gametes and diploid zygotes

1. Meiosis is the formation of haploid gametes and occurs in the testes/ovaries → produces sperm and egg 🥚

2. Meiosis halves the chromosome number (human germ cells with 46 chromosomes produce gametes with 23 chromosomes) 🥈

3. Fertilisation is the fusion of gametes/sex cells — means of inheritance, one from each parent, sexual reproduction 🪢

4. Because fertilisation involves the fusion of gamete the number of chromosomes in the next generation is doubled → diploid zygote is formed 👬

Outline the experimentation of Gregor Mendel

GREGOR MENDEL’S EXPERIMENT:

1. Conducted crosses between pea plants for physical traits: height, seed colour, and shape 🫛

2. He crossed yellow peas (true-breeding) with green peas (true-breeding) → all of the offspring (F1) were yellow 💛 💚

   • Yellow factor/allele is dominant—expressed

   • Green factor/allele is recessive — not expressed when combined with different allele

3. Cross between F1 pea plants + F1 pea plant → most were yellow, some green in a 3:1 ratio 💛 💛

   • alleles segregate and recombine randomly

OBSERVATIONS:

1. Traits were controlled by heritable factors (genes) in different forms (alleles) 🧬

2. Some forms of the traits were dominant (expressed and masked the other form) and some were recessive 💪

3. Traits were inherited independently from each other 🆒

4. Inherited forms of the trait segregate/separate during gamete formation and offspring randomly receive the factors: 😃

Compare the genotype and phenotype

GENOTYPE: Combination of alleles inherited by an organism, remains the same throughout organism life 🧬

PHENOTYPE: The observable traits of an organism resulting from genome and environmental factors 🙈

• Dominant alleles are expressed in the phenotype → masks the expression of recessive alleles

• Recessive alleles are expressed in the phenotype when BOTH alleles are recessive/homozygous recessive

1. The phenotype of an organism can change (plasticity)with the capacity to develop traits suited to the environment 🌍

2. Changes due to varying patterns of gene expression, but can be reversible ⭕

3. Influenced by: UV light, hormones, drugs, temperature, exercise 🏃

Outline phenylketonuria as an example of human disease

1. Phenylketonuria is a genetic/inherited disease due to a mutation in the PAH gene in chromosome 12 — PKU is due to a recessive allele ❤‍🩹

2. Affected individuals must receive the recessive allele from both parents 👪

3. PAH gene codes for the enzyme phenylalanine hydroxylate → converted amino acid phenylalanine to tyrosine 💻

4. Individuals with PKU have a build-up of phenylalanine which can result in seizures, involuntary muscle movements and brain damage 😥

Outline single-nucleotide polymorphisms

1. Single nucleotide polymorphisms are a single base change in DNA

2. SNP’s are created by base substitution mutations → creates new alleles

3. Only one allele can be a gamete, but there are two alleles in a genotype

Outline multiple alleles in gene pools

1. Some traits have multiple alleles, 3+ alleles

2. ABO blood types have multiple (IA, Ib, i)

3. Gene pools consist of all the genes and alleles in a population

Discuss ABO blood groups as an example of multiple alleles

MULTIPLE ALLELES: When a gene has 3+ alleles

1. ABO blood groups has three alleles (IA, IB, i)

2. Any two of these alleles are present in an individual

CODOMINANT ALLES: Both alleles are expressed

1. Produces a protein and effects the phenotype when present in a heterozygote

2. Combined as a heterozygote, the IA IB phenotype would be the AB blood type

3. The alleles IA and IB are co-dominant and both are dominant to i and are expressed as separate surface antigens on red blood cells

POSSIBLE GENOTYPES/PHENOTYPES:

1. Phenotypes are the blood groups: A, AB, O, B

2. Genotypes: IAIA, IAIB, ii, IBIB, IBi

Outline pollination

POLLINATION: is the transfer of pollen (male gamete) from one plant to the stigma of another plant

1. Stamen: filament holds the anther contains pollen — can be transferred by wind/insects

2. the transfer is called pollination

3. Pistil: Lands on the stigma, transfer down the style to the ovule/ovary)

4. Fertilisation occurs in the ovary (fusion of gametes)

Compare incomplete dominance and codominance

They are both examples of non mendelian inheritance (greater than 2 alleles)

INCOMPLETE DOMINANCE:

1. Dominant alleles don’t fully mask the recessive alleles

2. Heterozygotes have an intermediate phenotype

3. Ex. pink flowered plants x flowered plants — heterozygote has the intermediate phenotype of pink

CODOMINANT ALLELE

1. Both alleles are expressed/produces a protein

2. Affects the phenotype when present in a heterozygote

3. Ex. ABO blood groups

Outline the determination of sex in humans

1. Autosomes are the first 22 pairs of chromosomes — don’t determine sex

2. Sex chromosomes are the 23rd pair

   • X and Y

   • not entirely homologous (X is larger than Y)

   • determine the sex of an individual

3. The inheritance of Y sex chromosomes from the father provides the sry gene → initiates male determination

4. FEMALE:

   • sex chromosomes are XX

   • female chromosomes can have X chromosome in their egg

5. MALE:

   • sex chromosomes are XY

   • can have either X or Y chromosomes in their sperm

   • 50% chance the offspring will be female/male

Discuss an example of a sex-linked genetic disorder/sex linkage

SEX-LINKED GENETIC DISORDERS/SEX LINKAGE: when the gene is carried on a sex chromosome - either x or y chromosome

1. Haemophilia are examples of x-linked sex linkage

2. The inheritance of sex linkage is different in males than females — males are more likely to be affected in X-linked recessive disorders

3. Females possess two homologous X chromosomes and have three possible genotypes:

   • Normal: XHXH, XHXh

   • Colour blind: XhXh

4. Female carriers have two different alleles for X-linked recessive traits, and are heterozygous for the trait → women can be carriers if only one X chromosome is affected

5. Males have two possible genotypes:

   • Normal: XHY

   • Colour blind: XhY

6. Males have only one X chromosome → only one copy of the gene

Outline the continuous variation with an example

1. Continuous variation is due to polygenic inheritance and environmental factors

2. Polygenic inheritance is controlled by multiple genes — the more genes, the greater the variation

3. Normal distribution/bell curve in the phenotype

EXAMPLE: Skin colour

1. The dominant allele codes for the production of the pigment melanin

2. The combination of alleles determines the phenotype

3. Environment affects gene expression — greater UV exposure increase gene expression of melanin

4. the more recessive alleles there are, the lighter the skin colour

Define the following terms:

1. homozygous dominant, heterozygous, and homozygous recessive

2. Alleles, dominant allele, recessive allele

3. Carrier, loci, centromere

PP — homozygous dominant

Pp — heterozygous

pp — homozygous recessive

Allele: Different versions of a gene

Dominant allele: P — allele that is expressed in the phenotype

Recessive allele: p — allele that is masked, unless both are recessive

Carrier: Heterozygous carrier of a recessive disease-causing allele — has a recessive allele

Loci: Specific positions of genes on a chromosome

Centromere: Joins chromatids in cell divison

Define gene locus

1. Specific position of a gene/allele on a chromosome

2. Genes and alleles for the same gene have the same locus

Outline the recombination of alleles in linked genes

LINKED GENES: located on the same chromosomes, and the recombination of alleles is due to crossing over (genes have to be located far enough apart from each other)

1. Unliked genes are located on different chromosomes

2. Recombination of alleles is due to random orientation of homologous chromosomes and independent assortment of allele pairs

3. Possible combinations is 2^n

4. New combinations of alleles are produced in the gametes

5. Crossing over increases recombination and variation

Outline the idependent assortment of unliked genes in meiosis

1. 2+ genes/allele pairs are inherited independently of another → occurs in genes that aren’t linked/located on the same chromosome

2. Due to the random orientation of homologous chromosomes in metaphase I of meiosis I → line up at at the metaphse plate independently

3. homologous synapse forms a tetrad → tetrad formation occurs in prophase and has random orientation

4. Genetic variation increases with new allele combinations — creates 2^n, or 2^ 23 ( 8 million) possible combinations

5. Separation of homologous chromosomes in anaphase I creates allele combinations (segregations) → only one homologous chromosome in each gamete

Discuss the segregation of unlinked genes in meiosis

1. Unlinked genes segregate independently in meiosis

2. They are carried on separate chromosomes

3. Segregation of alleles occurring during anaphase I and II

4. Each allele pair separates and moves to the opposite pole

5. Only one allele for each gene/homologous chromosome is present in each gamete

What is the ratio of most dihybrid crosses?

9:3:3:1

Outline autosomal gene linkage

GENE LINKAGE:

1. Genes (2 genes) located on the same chromosome

2. Genes are inherited together/don’t assort independently

3. Non-medellian ratios

CROSSING OVER:

1. Occurs during prophase 1 of meiosis

2. Occurs between non-sister chromatids/homologous chromasomes

3. Creates recombination of alleles/recombinants — less common/frequent

4. Produces recombinant gametes

5. Occurs when genes are far apart more frequently

EXAMPLE: Lathryus odaratus, for genes for flower colour and pollen grain shame

Alleles:

P = purple

p = white

L = long

l = short

Parent genotype: PPLL x ppll

Gametes: PL x pl

Discuss the independent assortment of unlinked genes in meiosis

1. 2+ genes/allele pairs are inherited independently of each other — occurs in genes that are unlinked/not on the same chromosome

2. Due to the random orientation of homologous chromosomes in metaphase I of Meiosis I

3. Homologous chromosomes synapses and forms a tetrad during prophase I in random orientation

4. Increases genetic variation with new allele combinations, creating 2^ 23 or 8 million possible combinations

5. Separation of homologous chromosomes in Anaphase I creates allele combinations (segregations) — there is only one homologous chromosome in each gamete

Outline the segregation of unlinked genes in meiosis

1. Unlinked genes segregate independently in meiosis

2. Unlinked genes are carried on separate chromosomes

3. Segregation of alleles occurs in anaphase I (and anaphase II)

4. Each allele pair separates and moves to the opposite pole

5. Only one allele for each gene / homologous chromosome is present in the gamete

Discuss the recombination of alleles in unlinked genes

1. unlinked genes are located on different chromosomes

2. recombination of alleles is due to random orientation of homologous chromosomes and independent assortment of allele pairs

3. possible combinations of chromosomes is 2ⁿ, n=23, 2²³ = 8 million

4. new combinations of alleles are produced in the gametes

5. crossing over increases recombination and variation

Discuss the recombination of linked genes

1. Linked genes are located on the same chromosome

2. Recombination of alleles is due to crossing over

3. Genes have to be located far enough from each other

4. Offspring genotype is different than the parent genotype

5. Ratio of recombinations will be much lower and the same

How do you find the number of gametes and genotypes?

let n = number of genes

Gametes: 2^n

Genotypes: 3^n

Why is blood type o the universal donor? What blood types can O donate/receive

1. O is the universal donor because it has no surface antigens on red blood cells

2. O can donate to any blood type

3. O can only receive blood type O+/O-

Outline chi-squared test

X² = the sum of: [(observed - expected)²] divided by the expected

• determines whether it is statistically significant or if it is by chance

• look at the probability of 0.05

• degrees of freedom is the number of phenotypes minus one

• if x² is greater than the critical value at 0.05, the null hypothesis is rejected — there is a difference

• is x² is less than the critical value at 0.05, the null hypothesis is accepted — due to chance

Null hypothesis: no significant difference between observed and expected

Alternate hypothesis: there is a significant different between the observed and expected