GCSE Biology: Genetics - Inheritance.

 

You’ll need to learn some new Ke­y terms used in genetics and know: -

Genes exist in alternative forms

Monohybrid Genetic Diagram / Punnett squares and family pedigrees

Calculate and analyse outcomes from monohybrid crosses

Symptoms of sickle cell disease and cystic fibrosis

Pedigree analysis screening for genetic disorders

The sex of a person is controlled by one pair of chromosomes

How the sex of offspring is determined at fertilisation (genetic diagram)

How sex-linked genetic disorders are inherited

 

 

First you should recap Cell division, mitosis and meiosis…

on the cell biology page we covered the following: -

 

How do cells divide to become anything at all?

 

Through a process called… Mitosis!

 

Somatic (body) cells are described as Diploid (from the Greek meaning ‘double’). 

…you will see this written as “2n”

 

Mitosis is the process of normal cell division. 

 

In mitosis, the chromosomes are copied and divided equally between the 2 new daughter cells. ..

 

...So each mitotic division produces 2 cells…

 

SO... both are diploid, & each has exactly the same genes as the parent cell!! Mitosis produces identical cells

 

Some cells in the human body are not diploid... 

e.g. Gametes (sex cells) contain only 1 copy of each gene as they have only 1 set of chromosomes.

 

These cells are Haploid and are produced by a special type of cell division called Meiosis

 

A male and Female gamete join  together at fertilisation...  forming a.  zygote

This 1 cell then divides  by  Mitosis to produce   a complete  new   organism…

 

You received one chromatid from your father. 

 

And one from chromatid your mother!

 

So, although homologous pairs contain the same genes, they do not necessarily carry the same versions of each gene.

A different version of a gene is called an allele.

 

Remember... 

This is key to understanding why organisms vary!

 

which brings us to inheritance…

 

Remember… that in all living things, characteristics are passed on in the chromosomes that offspring inherit from their parents. 

 

 [Image of chromosomes - genes / alleles]

 

So, although homologous pairs contain the same genes, they do not necessarily carry the same versions of each gene.

 

 Remember that A different version of a gene is called an ALLELE.

 

so, Each chromosome may have a different version of a gene (an Allele). 

 

“Different versions of a gene, that code for different versions of a characteristic, are called ALLELES”. 

 

Ok, In genetics, different genes (alleles) are often represented by letters, such as Aa. 

 

The capital letter represents a DOMINANT allele ‘A’; whereas a lowercase letter ‘a’ would be the recessive allele.

 

some key terminology… Terminology is so important! Make sure you learn the language of biology - understanding what a word means makes all the difference! 

 

Here are some key terms you have to know: - 

Heterozygous, Homozygous (dominant and recessive), Phenotype, Genotype, Allele, Gene

 

Homozygous (dominant and recessive) and heterozygous…

 

Homozygous:

 

“Homo” meaning “the same” and zygous - referring to the zygote (the fertilised cell). So you can think of homozygous as having 2 of the same “letter” like 2 capital letters ‘AA’ or 2 lowercase letters ‘aa’.

 

Now you’ll know that a capital letter,  ‘A’ represents a dominant trait…. so 2 capital letters ‘AA’ can now be called Homozygous Dominant (2 of the same capital letters - which represent a particular trait, for example eye colour).

 

One of those A’s (genes/alleles) has come from mother whilst the other ‘A’ (gene/allele) has come from father - so, one of the letters represents the “sperm” and its genes, and the other represents the egg (ova) and its genes…. 

 

Homozygous individuals are true breeding. This means that they will always produce the same phenotype of offspring because they are not ‘hiding’ a recessive allele.

 

So what does heterozygous mean?

Well ‘hetero’ means “different” and again ‘zygous’ refers to the zygote (the fertilised cell).

 

So, in genetics and when we use letters to represent characteristics and inherited traits we can now think of heterozygous as being 2 different version of the same letter… so, a capital letter ‘A’ representing a dominant trait and a lowercase ‘a’ representing the recessive trait.

 

Aa = heterozygous

 

You know now that the “letters” ‘A’ and ‘a’ are the different version of the same gene - that is they are Alleles…

 

so we have:

 

Homozygous dominant: 2 of the same capital letters, AA, FF, HH, etc… to represent the 2 dominant genes inherited from each parent.

 

Homozygous recessive: 2 of the same lowercase letters, aa, ff, hh, etc… to represent the 2 recessive genes inherited from each parent.

 

Heterozygous: 2 different version of the same letters, ‘Aa’, Ff, Hh etc… to represent a dominant allele inherited from one parent and a recessive allele inherited from the other parent.

 

 

Now we know what the letters mean and the key terms homozygous dominant, homozygous recessive and heterozygous we can better understand what Gregor Mendel was up too when cross fertilising pea plants…

 

Mendel and Monohybrid inheritance: How single genes are passed on... Medal and his peas…

 

[images]

 

what is monohybrid? we can better understand this term like this: -

mono = one

hybrid = inheritance of a gene

 

so literally the inheritance of one gene…

 

ok, so… Monohybrid (single - gene) inheritance, is about the inheritance of different alleles BUT concerning a single gene. (e.g. the gene for height or the gene for flower colour)…

 

 

 

 

 

...Like Mendel, we’ll start with the pea plants, which have easily observable features that are controlled by a single gene;

 

for example Pea plants have one gene for height. 

 

But, the height gene has 2 alleles: 

 

you can represent the genes/alleles for height by using the letter ‘H’

 

You’’ll now that a capital ‘H’ (represents a Dominant allele, in this case the allele for being Tall). 

whilst the lowercase ‘h’ (represents a a recessive allele, in this case the alley for being short).

 

[images]

 

Pea plants are diploid and so have 2 alleles (for height). Therefore there are 3 possible genotypes:

 

1. The pea plant could have 2 Dominant alleles: “TT” which you know is called homozygous dominant (homo = same) (so you can say the pea plants are homozygous for T - Tallness)

 

2. The pea plant could have 2 recessive alleles: “tt” which you know is called homozygous recessive (homo = same) (so you can say the pea plants are homozygous for t - shortness)

 

3. The pea plant could have inherited 1 dominate allele and 1 recessive allele: “Tt” which you know is called Heterozygous (hetero = different).

 

What you must remember though is that a dominant allele will “overpower” a recessive  allele… so in this case Tt would be a Tall pea plant.

 

Note: more key terms!

The letters we have used, e.g. “Tt” are an organisms Genotype (the genes the organisms have inherited).

 

However, how the organisms looks - for example the heterozygous “Tt” pea plant looks Tall - this is its phenotype (the “Physical” outcome, or physical representation of the genotype)…

 

So the genotype is the genetic make up of an organism… the genes it contains, and you can use letters to represent them, such as using the letter “H” for height… HH, hh or Hh.

 

The Phenotype is the physical outcome - the expression of those genes… for example if the letter “H” is the dominate gene for Tall plant - you can physical see the organism, the pea plant in this case is Tall… but, you wouldn’t know if the pea plant was heterozygous or homozygous dominant (because you can’t see its genes!)

 

And this is exactly what Mendel was up to! (he wanted to know what genes the pea plants where passing on to their offspring after being fertilised).

 

 

 

 

Remember Pea plants are diploid - they have 2 alleles (for height in this example). 

 

So, Consider what happens when a homozygous Dominant plant (TT = Tall) is crossed with a homozygous recessive plant (tt = short).

 

[insert Punnett square]

 

All gametes from the Tall plant contain a T allele

All gametes from a dwarf plant contain a t allele. 

 

These pea plants combine at fertilisation to give offspring which have all inherited 1 Dominant gene (allele) ‘T’ from the homozygous dominate (Tall) plant, and 1 recessive allele from the homozygous recessive (short) plant ’t’. So, the genotypes for all offspring following this fertilisation must be “Tt” (hererozygous). and since the genes / alleles determine an organisms phenotype we now that all the pea plant “Look” Tall.

 

So we can say The 1st generation of pea plants, (known as the F1 generation), are all Tall because Tallness is determined by the Dominant Allele (T)…

However, although the pea plants all look identical (and just like the Tall parent plant), they are very different in one very important respect: they are Heterozygous and NOT homozygous!

 

Remember: How an organism actually looks is its called its phenotype!

 

Mendel wanted to know: What happens when 2 of these heterozygous pea plants are fertilised? how will they look?

 

So, If 2 of these heterozygous plants are crossed, half of the gametes from each parent are T and half are t, giving us 4 outcome with 3 possible genotypes in the second generation (F2)…

 

the genotypes are: -

 

1. HH, 

2. Hh, 

3. Hh and 

4. hh.

 

[insert Punnett square]

 

The first 3 genotypes are all Tall plants (phenotypically), but the fourth is a short plant (phenotypically). You can say 75 % are Tall and 25% are short…

 

 

Monohybrid inheritance in humans

 

Clear cut examples of monohybrid inheritance in humans are pretty rare, but often they involve a genetic disease where people inherit 1 or more faulty alleles. 

 

Genetic diseases are often recessive; this is because faulty alleles that fail to make an important protein can be masked by normal ones that function properly.

 

i.e. Recessive alleles are overpowered (masked) by Dominant Alleles.

 

In contrast though, some genetic diseases like Huntington's disease can be caused by DOMINANT ALLELES. The alleles concerned code for a product that actively causes damage; symptoms are not due to an allele not doing its job. Such alleles are dominant because the presence of a normal allele cannot mask the symptoms.

 

Some examples of monohybrid inheritance in humans: -

 

[insert image/table of monohybrid inheritance]

 

Pedigree analysis screening for genetic disorders)

 

Huntington's disease is a rare inherited disorder of the nervous system. it is caused by a dominant allele (we’ll represent this allele with the capital letter ‘H’. The recessive allele for this gene we can represented by lowercase letter ‘h’.

 

[insert “Pedigree” image]

 

The diagram shows the inheritance of Huntington’s disease in a family.

 

Use a genetic diagram to show the inheritance of the Huntington's Disease allele by the children of parents P and Q.

 

[images of genetic diagrams]

 

Q) Explain why none of the children of R and S inherited Huntington’s disease.

 

[insert image for Q above]

 

A) Both parents are unaffected; They do not have Huntington’s disease. 

The Genotype of the parents is... hh homozygous recessive (So, neither parent has the ‘H’ dominant gene/allele - therefore cannot pass on the disease).

 

Calculate and analyse outcomes from monohybrid crosses

What if both parents are heterozygous? Organise your results into a Punnett square: -

 

[insert images of Punnett square and explainations]

 

Monohybrid Genetic Diagram Punnett squares and family pedigrees…

Cystic fibrosis is a recessive disorder. Cross 2 carriers with the  genotypes Ff

 

[insert genetic diagram images]

 

Organise the results of the genetic cross into a Punnett square: -

 

[insert Punnett square / plain ratio / %]

 

 

 

You know that Meiosis is a special type of cell division which results in the production of Gametes (sex cells). 

 

“Meiosis is a special kind of cell division in which there are 2 successive divisions that result in the production of gametes – the sex cells!”

 

Remember sperm and egg (ova) are Haploid – having just “n” half the number of chromosomes.

 

Meiosis: The production of Gametes (sex cells)  

 

You also need to know that: -

The sex of a person is controlled by one pair of chromosomes (X and Y).

How the sex of offspring is determined at fertilisation (use a genetic diagram).

 

[insert genetic diagram -showing inheritance of X and Y]

 

Organise your results into a Punnett square: - 

[insert Punnett square showing inheritance of X and Y]

 

Sex linked Genetic disorders… How sex-linked genetic disorders are inherited…

 

You know who the sex of a person is determined by the inheritance of their X and Y chromosomes, now you need to know that Some genetic characteristics are also “sex linked” – meaning they are carried on one of the Sex Chromosomes (X or Y). 

 

Colour Blindness is and example of an inherited sex linked characteristic… it is caused by a faulty allele on the X chromosome… 

 

Since the Y chromosome is smaller than the X chromosome it carries fewer sex-linked genetic disorders! 

 

So, colour blindness, for example is much more common in men than in women – because men only require 1 (recessive) faulty allele – whereas women need 2.

 

[insert images: This genotype is rare, This genotype is more common]

 

[insert images of genetic diagram showing inheritance of colour blindness - sex linked]

[insert images of Punnett square showing inheritance of colour blindness - sex linked]

 

 

Sex linked Genetic disorders: 

 

Haemophilia (a disease where blood doesn't clot properly)  is a genetic disease that is inherited in exactly the same way as colour blindness.

 

Haemophilia is inherited, and is caused by a faulty allele on the X chromosome…

 

 

 

Recap/revise key words: - 

 

Homozygous: -

Pair of alleles that produce a characteristic are the same, e.g. 

Homozygous dominant = HH

Homozygous Recessive = hh 

 

Heterozygous: - Pair of alleles that produce a characteristic; the alleles are different, e.g. Hh. 

 

Dominant: -

An allele that will always be expressed even when there is only one of these alleles present, represented by a capital letter. e.g. HH or Hh. – the H (dominate allele – will be expressed) 

 

Recessive: - An allele that will only be expressed when both alleles are of this type e.g. hh. 

 

Gene: - Section of DNA that codes for a particular trait or characteristic. 

 

Allele: - A different form of a gene that codes for a different version of the same characteristic (i.e. different eye colour) 

 

Genotype: - A description of the pair of alleles present for a characteristic. 

 

Phenotype: - The physical expression of the alleles.