Mutations 2

Key words Monoploid Parthogenesis Polyploid Aneuploid Non-disjunction Trisomy monosomy

Haploid and Diploid

Monoploidy A cell organism that has a functional genome consisting of one copy of each chromosome, represented by 1n Male bees are monoploid (1n), while females and queens are diploid Monoploid does not mean haploid – male bees monoploidy status give them a single operational set of chromosomes Haploid gametes are waiting (dormant) to be fertilised , which activates them. This results in diploid female offspring Males are produced by parthenogenesis – a process by which entire organisms are regenerated from a single egg cell without the need for fertilisation

Monoploid and Diploid Monoploid exists in many organisms e.g. fungi, fish, amphibians and reptiles Efficient, but majority of organisms are diploid – why? Any defective alleles are masked by a functional allele on a corresponding chromosome. In monoploid organisms cannot do this

Polyploidy Cell division during meiosis results in haploid cells When spindle fibers fail, some cells contain double the genetic information, while others contain none at all  See diagram p77

Polyploidy Polyploidy is common in flowering plants, ferns and green algae Fruits and cereals have been genetically manipulated to be polyploidy for human application e.g. seedless fruits, larger sized fruit Diploid (2n) fusing with haploid (n) produces a triploid organism (3n) – three of each type of chromosome Polyploidy is lethal in humans 

Aneuploidy A condition where there is an addition or loss of one chromosome from a cell e.g. 2n + 1 or 2n – 1 Results from non-disjunction

Non-disjunction During meiosis, two homologous chromosomes, instead of separating, both go into the same gamete. The spindle fibers have failed. This is known as non-disjunction. As a result of non-disjunction some gametes have two copies of a chromosome and others lack a copy. After the first meiotic division the homologous pair, instead of separating, have both remained in the same cell. After the second meiotic division the gametes formed are of two types; half lack a chromosome and half have two copies of the same chromosome.

Down’s Syndrome If non-disjunction occurs in a human egg cell then one or more abnormal eggs may be formed This means the haploid number of these abnormal eggs is 24 If this egg is fertilised the diploid number of the zygote is 47 instead of 46 An extra copy of chromosome 21 is passed on as a result. This can be clearly seen in a karyogram and is known as trisomy 21 Sufferers of Down’s syndrome in general have learning difficulties, heart problems and distinctive facial features

Non-disjunction of Sex chromosomes Females: Turners Syndrome Sufferer is short in stature and show a lack of sexual development at puberty This non-disjunction results in only one X chromosome: XO

Non-disjunction of Sex chromosomes Males: Kleinfelter’s syndrome Sufferers are males with tests that remain very small. Few sperm are produced therefore they are mostly infertile and low levels of testosterone are produced leading to reduced development in puberty This non-disjunction results in only one the individual being XXY

Genetic Screening and Counselling – FYI only! Genetic counsellors aim to assess the risk of parents passing on genetic diseases to their offspring so that they can make informed decisions Two options are pre and postnatal screening

Prenatal Screening – FYI only! Chorionic Villus Sampling (CVS) A high risk procedure done early on in pregnancy Sample of developing placenta is taken Amniocentesis Amniotic fluid which foetus bathes in is extracted Cells obtained are photographed and the karyogram is analysed for any abnormalities

Genes and Body plans How does a zygote become an embryo Genes and Body plans How does a zygote become an embryo? How do the different parts of an organism know which way is “up”?

Learning Outcomes Explain that the genes that control development of body plans are similar in plants, animals and fungi, with reference to homeobox sequences (HSW1); Key words… Homeobox genes Transcription factor Homeosis Polarity genes Body plan Morphogen Maternal-effect genes Segmentation genes

What do homeobox genes do? Homeobox genes code for the production of transcription factors These proteins can bind to a particular region of DNA and cause it to be transcribed A single homeobox gene can switch on a whole collection of other genes, regulating gene expression

Homeobox genes Homeobox genes determine how an organism’s body develops as it grows from a zygote into a complete organism. They determine the organism’s body plan These sequences are highly conserved Homeobox genes have been discovered in animals, plants and fungi

Homologous homeobox genes These are the sequences of 60 amino acids in the proteins coded for by the homeobox genes Antp in a fruit fly and HoxB7 in a mouse. All animals have homologous homeobox genes – they are recognisably similar. Why is this significant?

Key words – homeobox, maternal effect and polarity genes What are they Key words – homeobox, maternal effect and polarity genes What are they? What do they mean? Homeobox genes control the development of the body plan of an organism, including the polarity (head and tail ends) and positioning of the organs Maternal effect genes determine the embryo’s polarity Polarity genes Help to define the anterior and posterior polarities within each embryonic segment of an organism, such as in the fruit fly

Homeosis - when one part of a developing embryo becomes anomalously transformed into another (mutation)

Name the type of gene, if mutated, gives rise to dramatic changes in body plan Homeobox gene

Drosophila melanogaster – the fruit fly A model animal for the study of genetics and gene regulation The body of insects is segmented This is obvious in larvae (e.g. maggots) In adults these become specialised -many develop special appendages

Head Segments - antennae - labial palps (mouth appendages) Thoracic Segments - legs - wings - halteres (balancing organs) Abdominal Segments - no appendages Identity of each segment is established in the embryo Mutations can destroy the identity of a segment (more on this later)

Homeosis in Drosophila Wings for halteres Legs for antennae These changes are the results of mutations in a set of genes (homeosis) In this instance, are mutations a good thing? Halteres for wings

Homeobox genes have been found in: worms snails starfish fish mice humans

Summary The body plan of the drosophila fly is controlled by the homebox genes. These contain transcription factors that regulate other genes so that the body plan develops normally. Transcription factors act like on/off switches for gene expression. Some of these genes are maternal-effect genes. They will determine the polarity of the embryo. This means which end is the front and which is the back. Segmentation genes are another group of genes that determine the dorsal and ventral ends of specific segments of the fly’s body.

Sex Determination SRY gene on the Y chromosome directs the development of male characteristics in mammals SRY gene activates DNA to express the genetic programme for testes development Consequently, testosterone and penis development as a result

X Chromosome Inactivation X Chromosome inactivation restores the proper concentration of proteins within cells of females and the process that leads to further variation in gene expression Two X chromosomes produce twice the chemicals required – disrupts biochemistry X inactivation is random, therefore increases variation