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(a) Genetic variation and human reproduction

Know and understand that sexual reproduction gives rise to variation because, when gametes fuse, one of each pair of alleles comes from each parent.

Know and understand that in human body cells, one of the 23 pairs of chromosomes carries the genes that determine sex.

All human cells have 22 matched pairs of chromosomes but the 23rd chromosome is different between the sexes.

Diagrams of chromosomes from micrographs

(i) In the above diagram the pairs of chromosomes are shown joined together by a centromere during duplication to give the X shape. (image adapted from shutterstock.com 701025034) e.g. see the cell division by meiosis diagram below.

(ii) In this diagram the pairs of chromosomes are shown as separate chromatids. (image adapted from the US National Library of Medicine) This profile of a set of chromosomes is an example of a karyotype.

22 pairs of the chromosomes look the same in both males and females and most are roughly X shaped when one is copied.

They are numbered 1 to 22 in decreasing size. However, for the 23rd pair of sex chromosomes, men have an X and Y chromosome (XY on the diagram) and women have two X chromosomes (XX on the diagram)

The lack of the Y chromosome, i.e. the XX gene combination causes female characteristics to develop in the embryo, eventually producing an adult female.

The Y chromosome carries a gene that causes male characteristics to develop in the embryo, eventually producing an adult male.

Male cells in the testes and female cells in the ovary divide by meiosis - illustrated below,

Diagrammatic reminders of sexual reproduction including meiosis and fertilisation.  For more details on meiosis see Cell division - cell cycle - mitosis, meiosis, sexual/asexual reproduction, binary fission  gcse biology revision

In sexual reproduction, the parents (mother and father) produce gametes (egg and sperm reproductive cells).

Each gamete only has one copy of each chromosome, unlike pairs of chromosomes in all other cells.

Therefore the gametes have only one version of each gene, an allele.

In producing offspring from fertilisation, the chromosomes from a male gamete (sperm) mix with the chromosomes from the female gamete (egg) to produce the full compliment of pairs of chromosomes - two alleles for each gene.

When sperm is made the X and Y chromosomes are drawn apart in the first meiotic division.

Therefore, in the first stage of the meiosis of sperm cells, there is a 50% chance of having an X or Y chromosome in the new sperm cell.

All egg cells will always have one X chromosome.

Therefore on egg fertilisation there is a 50% chance of an XX or XY combination ie a 50% chance of being male or female (see table and diagram below).

Note use of the word 'chance'. These 'chances' are the probable outcome of many sexual reproductions.

In any data set, because of the random combinations of the gametes (from available possibilities), the outcome is unlikely to be perfectly 1:1, but more likely 48% : 50% (0.48 : 0.52) or 51% to 49% (0.51 : 0.49)

So bear this idea in mind when ratios like 1 : 3 etc. are quoted i.e. in reality as well as the possibility of 1.00 : 3.00, for other data sets it might be 0.97 : 3.03 or 1.02 to 2.98).

Tabular and diagrammatic methods of obtaining these probability ratios are described in section (b).

Footnote

My good Irish wife Molly, had a cousin who has seven sons and no daughters!

So much for statistical probability and the apparent dominance of the XY genotype here!

 

Note: In a fertilised egg, multiple cell divisions occur by mitosis to produce all the huge number of cells a complex living organism like ourselves needs to grow and develop.

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(b) Revision - Methods of constructing two types of genetic diagrams

1. Punnett square genetic diagram for determination of gender

To find the probability of phenotype outcomes you can construct a Punnett square deduced from 'crossing' the different genes or chromosomes.

In this case you construct a genetic diagram or 'chart' to show the possible outcomes from XX crossed with XY.

You put the possible gametes from the female above the ('yellow') square (X and X) and the possible male gametes (X and Y) down the left side of the square.

You then fill in the matching genotype pairings giving XX, XX, XY and XY.

As you can see, on average there are two male phenotype and two female phenotype outcomes.

In other words, a 2 in 4 (50%) chance of a baby being a boy or a girl.

These outcomes can also be shown as another type of genetic diagram shown below.

2. Circles with connecting lines genetic diagram for determination of gender

You can also construct a 2nd type of genetic diagram using circles and connecting lines.

At the top are the parents indicating the phenotype and genotype.

Below that you show the possible gametes that can be formed, X or Y.

One gamete from parent a combines with one gamete from parent b in fertilisation.

You then use connecting lines to show how the chromosomes can combine, XX or XY.

Finally, the bottom row of circles show the genotypes of the offspring, to which you can add the phenotype, XX = female and XY = male.

For more on genetic diagrams see

Introduction to inheritance of characteristics and genetic diagrams (including Punnett squares)  including technical terms, Mendel's work, inherited genetic disorder, genetic testing

More complicated genetics: Sex-linked genetic disorders, inheritance of blood groups

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(c) Genetic fingerprinting

Reminders

Know and understand that some characteristics are controlled by a single gene.

Each gene may have different forms called alleles.

Know and understand that an allele that controls the development of a characteristic when it is present on only one of the chromosomes is a dominant allele.

This is important when interpreting genetic diagrams (see above with the genetic disorder polydactyly).

Know and understand that an allele that controls the development of characteristics only if the dominant allele is not present is a recessive allele.

This is important when interpreting genetic diagrams (see with the genetic disorder cystic fibrosis).

Know and understand that a gene is a small section of DNA.

Genes code for specific proteins and the type of cell they form part of.

Know and understand that each gene codes for a particular combination of amino acids which make a specific protein.

Know and understand that each person (apart from identical twins) has unique DNA - a genetic fingerprint.

DNA fingerprinting is a technique that simultaneously detects lots of sections in the human genome to produce a pattern unique to an individual.

This is a DNA fingerprint and the probability of having two people with the same DNA fingerprint that are not identical twins is very small indeed.

(Actually, because of the chance of imperfect DNA replication, even identical twins don't have a perfect match of their whole genome - but the phenotype outcomes are so close, the term 'identical twins' is still appropriate, since it is difficult to detect their differences.)

Know that this can be used to identify individuals in a process known as DNA fingerprinting.

The technique is used in forensic science and your DNA can be checked against a database of previous suspects or convicted criminals!

It is also used in archaeology to try and establish the original of ancient bodies and bones!

All you need is a sample of blood, hair, semen or skin from a body or crime scene.

It can also be used to identify if an individual is a relative of another.

As I was working on this page in 2013, the bones of King Richard III had been found by archaeologists in the City of Leicester, England. Chromosomal DNA was extracted from the bones and compared with one of the few known descendents of his family (a man in Canada, I think?) and a family match established. The bones showed that Richard III had a deformed back ('hunchback'), but you didn't need DNA to confirm that!

Since writing the above paragraph. on re-visiting Leicester, I took a photograph of the DNA evidence for confirming the bones found were those of Richard III (image below from the exhibition in the medieval Guildhall in Leicester from the work done by Leicester University).

They compared the mitochondrial DNA of Michael Ibsen and a 2nd matrilineal (lineage 2), with that of DNA extracted from the bones of Richard III. See the diagram below.

You can see the matching base peaks (colour coded) for the specific and characteristic sequence based on the four bases G (guanine), A (adenine), C (cytosine) and T (thymine) found in the structure of the compared DNA molecules of the individual genomes.

The sequence reads in sections such as ...GAACAAGCTATGTA.... etc.

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(d) Learning objectives for this page

• Be able to demonstrate an understanding of the implications of sequencing the human genome (Human Genome Project) and of the collaboration that took place within this project.
  • The project has mapped the DNA sequence for the ~25,000 genes of the 23 pairs of chromosomes from human cells.
    • By getting many genetic research groups to collaborate and work together on the project simultaneously e.g. sharing out the genes/chromosomes to be analysed between them, it became much quicker to produce the full human genome sequence.
  • What is the point of the Human Genome Project? What can we gain from it?
    • We are gradually building up a database of which genes ('genetic character') that predispose people to particular conditions.
    • Therefore, it may enable us to predict which people are likely to suffer from a particular disease or disorder and therefore perhaps offer a preventive course of action, which may involve medical treatment or lifestyle changes.
    • It may be possible, using genetic engineering, to prevent diseases such as cystic fibrosis and sickle cell anaemia.
    • Could we produce 'designer medicines' based on our own genetic blueprint?
    • Can we develop more accurate diagnostic techniques for certain conditions which are difficult to diagnose at an early stage?
    • Each person has unique and characteristic DNA sequence, and genetic fingerprinting is already being used to identify bodies, suspects and innocent people by forensic scientists.
      • It is also used by archaeologists too.
      • Will it be possible in the future to even get a more detailed picture of a person just from a DNA sample? e.g. hair/skin colour, eye colour and other body characteristics?
    • So far, all positive possibilities, so is there a downside to the Human Genome Project?
    • I'm afraid so, although its great science, the social implications of this genetic knowledge raise serious ethical issues about what is acceptable to society.
      • If it is known that you may be susceptible to a particular disease or disorder which you may suffer from later in life, what happens if your employer, medical insurance company or life insurance company has your genetic profile?
      • You could be discriminated against, e.g. an insurance company may demand your genetic profile and modify the premiums you pay according to your 'genetic risk'.
      • This may not be the only thing that bothers you, if are told that you may suffer from a particular disease or disorder, you may be worried about or perhaps undertake preventative courses of action which may not be required?