B1 Revision - You and Your Genes - Science Department
B1 Revision You and Your Genes You and Your Genes (B1) Revision for Exam What makes us all different? Organisms inherit information from their parents. This controls how they develop, so children look a lot like their parents. People share many common features and the differences between people are very small. These differences make us unique. Chromosomes are Most features are also affected by the made of DNA environment e.g. Weight, height, skin molecules colour All living things are made of cells Different proteins in the body do different jobs. H: They can be structural (to build the body) or enzymes (to speed up chemical reactions in the body) Genes are instructions which tell the cell how to
What makes a person male or female? A fertilised human egg cell contains 23 pairs of chromosomes. Pair 23 are the sex chromosomes. Females have two X chromosom es - XX Males have an X and Y chromosom H: The SRY gene on the Y chromosome causes e - XY the testes to produce the male sex hormone, androgen, which makes the embryo develop into a male. sperm male body cell or female body cell 50% chance that a baby will be a boy or a girl 50% chance that an X or Y chromosome fertilises an egg
all egg cells Inheritance and Alleles Parents pass on genes in their sex cells, which contain copies of half the parents chromosomes. Alleles = different versions of genes in a pair Dominant -in charge -only need 1 copy to have its feature same gene Chromosomes come in pairs. They carry the same genes in the same place on each chromosome, so genes
also come in pairs sex cells get one chromosome pair chromosome from each pair the parent has Recessiv cannot predict which e egg & sperm cells will - must Example: meet have 2 T = dominant allele for straight thumbs copies of t = recessive allele for curved thumbs this allele TT = straight thumbs Tt = straight thumbs tt to have its = curved thumbs Huntingtons Disorder Symptoms (Affects the nervous system, later in life e.g. 40-50yrs old) -forgetful difficulty understanding things -twitching of muscles unable to control movements -eventually a fatal condition
An inherited condition, caused by a dominant allele Worked Example: H = dominant allele for Huntingtons disorder h = recessive allele (non-Huntingtons) If the man is Hh, he will get the disorder. If the woman is hh, what percentage of their children will inherit the condition? father Hh Mother hh h h H h Hh Hh Hh hh Hh affected, so, 50% will inherit Huntingtons Cystic Fibrosis Caused by a faulty recessive allele
(f), so a person may be a carrier of the faulty gene (Ff), but not have cystic fibrosis 1 in 25 people in the UK carries the faulty recessive allele. Would you want to know if youre a carrier? breathlessness chest infections food not digested properly short of nutrients If both parents are carriers there is a 1 in 4 chance their child will have CF father (carrier) Ff mother (carrier) Ff CF is an inherited illness -affects breathing & digestion -cells that make mucus produce mucus which is too
thick, so it blocks the lungs & tubes that take enzymes from the pancreas to the gut F f F FF Ff f Ff ff Asexual and Sexual Reproduction Asexual reproduction is performed by only one parent & the offspring are genetically identical to the parent. They are clones. Any variation between them is caused by differences in their sperm
environment baby Plants keep some unspecialized splits cells all their lives, fertilised which can become egg baby anything that the plant might need. egg They can be used Most animals use sexual reproduction, requiring 2 parents & for asexual producing offspring which contain a random mix of genes from both parents. Clones are sometimes produced when identical reproduction. twins are formed. These are natural clones. Artificial clones can be made (Dolly) Genetic Testing of Adults Genetic testing is when an individual is tested for the presence of a particular allele that may cause a genetic disorder. Who would want to be tested?
Like Huntingtons disorder or cystic fibrosis? Are there any disadvantages? Someone with a family history for a serious genetic disorder. Yes. It helps them to plan their lives. Such as whether to have children of their own or not. Yes. Testing can be stressful and in some cases, a positive result may make it difficult to get a mortgage, job or life insurance. Genetic testing of foetuses A couple may decide to have a genetic test on the foetus during pregnancy, if they know there is a risk they may pass on a serious genetic disorder to their children. How is this done? What happens next?
Are there any risks from testing? Doctors collect cells from the foetus during pregnancy, using an amniocentesis test, and then examine the genes of the foetus. If the child will inherit the genetic disorder, the parents may choose to have a termination (abortion). Yes. There is a risk of miscarriage during the amniocentesis test and the results are not 100% reliable. Genetic testing of embryos A couple may decide to use pre-implantation genetic diagnosis (PGD), to avoid passing on a serious genetic disorder to their children, without the possible need for a termination. The woman is given fertility drugs, so she produces several ova at once. The ova are collected and mixed with sperm in a dish. Fertilization occurs. This is in vitro fertilisation (IVF). The fertilised eggs start to develop into embryos. Cells from the embryos are tested for the presence of faulty alleles that cause the genetic disorder.
Only embryos without the faulty alleles are implanted into the uterus of the woman to develop into a baby. PGD can only be used for families with certain inherited conditions. The Human Fertilisation and Embryology Authority (HFEA) is a committee that decides when PGD can be used. Genetic Screening Genetic screening is when a whole population is tested for a particular allele. The NHS and Government decide whether to use genetic screening based upon: the relative costs of screening compared to treatment the possible benefits to individuals and the population Who should have access to the information? Employers Individuals Assess if employee will Make informed be fit to work in the decisions. future. May not want to know. Insurance Companies May lead to Right to choose. discrimination of
Assess if person is a individuals. higher risk. Charge higher premiums or refuse insurance. Gene Therapy Gene therapy involves introducing normal functional alleles into the cells of patients with a genetic disorder. In the future, it is hoped gene therapy will cure genetic disorders such as cystic fibrosis. Find the gene that causes the disorder on a chromosome. Cut out the normal allele from another chromosome and make lots of copies of it. Introduce the normal alleles into cells of the patient. Therapeutic Cloning Stem cells are unspecialised cells that can develop into different cell types. Scientists
believe stem cells could be used to treat certain degenerative diseases e.g. Parkinsons. Embryonic stem cells are most useful because they can develop into any cell type found in the body. Nucleus removed from egg cell. Egg cell triggered to develop into an embryo. Embryo Stem cells are taken from the embryo and treated so they develop into different cell types e.g. nerve cells that are used to treat the patient. Nucleus from patients body cell inserted into These cells have the same genes as the patient, so would not be rejected.
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