TV4 - Ch.19: The basics of hereditance

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• Middelbare school
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• Leerjaar 4

The basics of heredity

TV4

Chapter 19

Hereditance

Study the image
How is this particular type of image called?

1. chromosome map
2. chromogram
3. karyogram
4. allele passport

Study the image (again)

What is true?

1. This person is male
2. This person has no chromosomal abnormalities
3. This person has too few chromosomes
4. This person has too many chromosomes

A point mutation in a gene may result in a change in the gene product (protein). THis is because a codon might have changed, resulting in the incorporation of a different amino acid in the protein structure.
Explain if a karyogram could be used to detect such mutations.

Chromosomal abnormalities like tri- or monosomies are caused by errors occurring during meiosis 1 and/or meiosis 2. How are these errors in chromosomal distribution called?

1. Genetic recombination
2. Genetic modification
3. Non-disjunction
4. Diploidy

How many chromosomes are in the human (nuclear) genome?

1. 23
2. 46
3. 92
4. 256

What does diploid mean?
How many copies are present of each chromosome?
Only note a number.

Learning objectives

• You acknowledge that genes are located on fixed locations within the genome
• You can describe the difference between a gene and allele
• You can explain how a difference allele dominance may result in difference in phenotype
• You can explain how alleles are distributed in gametes

Genes, alleles, loci and chromosomes

A gene encodes a protein. These proteins will result in having certain (hereditary) traits. 

Every gene is located on a fixed spot on the genome. This spot is called a locus (pleural: loci).

Each gene might have different variants. A gene variant is called an allele.

Because a diploid organism has two copies of each chromosome (homologous), this organism will have two loci of each genes. These may carry the same or different alleles. 

Drag the analogous description to the proper term

A gene variant: because of mutations, differences might have emerged. Because of these differences, proteins have diverged.

A nucleotide sequence that contains conding information, used by ribosomes to synthesize proteins. 

Because both the father and mother have donated a copy, an organism has two of these. Together they belong to the same pair. There are small genetic differences between the two. 

A fixed locus on a chromosome. On this spot each organism (of the same species) has the same genetic information that, although small difference are present. 

homologous chromosomes

allele

locus

gene

Genes, alleles, loci and chromosomes

A gene encodes a protein. These proteins will result in having certain (hereditary) traits. 

Every gene is located on a fixed spot on the genome. This spot is called a locus (pleural: loci).

Each gene might have different variants. A gene variant is called an allele.

Because a diploid organism has two copies of each chromosome (homologous), this organism will have two loci of each genes. These may carry the same or different alleles. 

Two alleles per locus or gene

Two of the same alleles?  HOMOZYGOUS

Two different alleles? HETEROZYGOUS

Study the image

What is true?

This organism is...

1. homozygous for the left gene heterozygous for the middle gene heterozygous for the right gen
2. heterozygous for the left gene heterozygous for the middle gene homozygous for the right gene
3. homozygous for the left gene homozygous for the middle gene heterozygous for the right gene
4. homozygous for the left gene heterozygous for the middle gene homozygous for the right gene

Dominance

In many cases one allele is dominant. This means that when an individual is heterozygous, only the dominant allele will be determining the phenotype. 

The other allele is called recessive.

A black guinea pig male is bred with a white guinea pig female. They get 20 individuals as offspring which are all black
What allele is dominant? Explain why.

Chinchillas (guinea pig resembling vermin) do it differently. A black and a white specimen are bred and they get numerous offspring. In contrast to guinea pigs, all of these off spring are dark grey.
Give a possible explanation for this different phenotype.

Letter notation for genes en alleles

Complete dominance

The letter is assigned to the gene *

dominante allele: capital letter   A

recessive allele: small letter   a

Genotypes

homozygous dominant:    AA

heterozygous:                       Aa en aA**

homozygous recessive:      aa

* = if you choose a letter yourself, choose a letter that has a different capital letter than small letter.

** = the capital letter MUST be noted down first.

A pea plant that has green seeds (peas) is bred with a plant with yellow seeds.

ALL offspring have green peas
Wat is the genotype of both parent plants?
Wat is het genotype of the offspring?
Use the letter K and/or k to indicate the alleles.

Incomplete dominance

Some times the recessive allele also influences the phenotype just slightly

Some times both alleles are evenly dominant in determining the phenotype

A homozygous plant with two alleles for red flowers have red flowers.

A homozygous plant with two alleles for white flowers have white flowers.

A heterozygous plant with both an allele for red flowers and for white flowers actually have pink flowers.

Because both alleles are evenly dominant, both are determining the phenotype together.

The resulting phenotype is called an intermediate phenotype.

Letter notation for genes and alleles

Co-dominance

A letter is assigned to the gene

Because both alleles are evenly dominant, both alleles get the same capital letter. A second letter is needed to indicate the difference in alleles (in superscript)

BR = allele for red flowers

BW = allele for white flowers

Genotypes

plant with red flowers:    BRBR

plant with white flowers:  BWBW

plant with pink flowers:    BRBW and BWBR

A snapdragon (plant) with red flowers is bred with a snapdragon with white flowers.
ALL offspring have pink flowers.
What is the genotype of both parent plants?
What is the genotype of the offspring?
Use the letter BR and/or BW to indicate the alleles.

If your ear lobe is attatched to your head or not is a hereditary trait that is monogenetic. It is caused by one single gene. The allele for loose lobes is dominant.

What gametes will a heterozygous father produce?

1. 100% of his spermcells are Aa
2. 50% of his sperm cells are A 50% of his sperm cells are a
3. 75% of his sperm celles are A 25% of his sperm cells are a
4. 100% of his sperm cells are a

Segregation of alleles during meiosis

Een somatic cell  is diploīd

(2n) so every gene is present twice         (Pp)

A gamete is haploid (n) so every gene is present once. (P)

During meiosis 1 homologous pairs are split. Half of the daughter cells will receive one allele, the other half the other allele. 

So a cell with genotype Pp that undergoes meiosis will result in having 50% of the gametes being P and the other half being p. 

A father that is heterozygous for the ear lobe gene gets a child with a mother that is also heterozygous.

What gametes do these parents produce?

1. dad: 50% A and 50% A mom: 50% a and 50% a
2. dad: 100% Aa mom: 100% Aa
3. dad: 50% A and 50% a mom: 50% A and 50% a
4. dad: 75% A and 25% a mom: 75% A and 25% a

Punnett squares

gameten of the parents along the axes of the table

in the table cells: possible genotypes of the offspring

Example

chance that a sperm cell with the recessive gene fertilises an ovum is 1/2

chance that the ovum itself has the recessive allele is 1/2

chance that zygote has both recessive genes therefore is 1/2 * 1/2 = 1/4

Do the following assignment on paper.
Take a picture of your answer and upload it here.
A man that is heterozygous for the ear lone gene has a child with a woman that has attached (recessive) earlobes.
Calculate the chance that the child also has attached lobes. Explain your answer with a Punnett square.

Learning objectives

• You acknowledge that genes are located on fixed locations within the genome
• You can describe the difference between a gene and allele
• You can explain how a difference allele dominance may result in difference in phenotype
• You can explain how alleles are distributed in gametes