Biology & Geology 4th ESO - Human Genetics PDF

BIOLOGY & GEOLOGY 4th ESO Unit 4: Human genetics 1. The human Karyotype 1.1. Karyotype The karyotype is the set of chromosomes of a cell or of an individual.  Homologous chromosomes: pairs of chromosomes....

BIOLOGY & GEOLOGY 4th ESO Unit 4: Human genetics 1. The human Karyotype 1.1. Karyotype The karyotype is the set of chromosomes of a cell or of an individual.  Homologous chromosomes: pairs of chromosomes.  Cell types vary according to the number of chromosomes: ◦ Diploids (2n): cells of the body (somatic). ◦ Haploids (n): reproductive cells. 1.2. Human karyotype The human karyotype is different considering the cell type:  46 chromosomes in somatic cells. ◦ 22 pairs of autosomes: homologous chromosomes. ◦ 23 pair of sex chromosomes: X, Y.  23 chromosomes in sex cells. 2. Inheritance in humans 2.1. Human genetics and inheritance Human genetics has difficulties associated to its study:  Few descendants per pair.  Long generation time.  Crosses can’t be planned (because of ethical reasons). The main causes of genetic variability are:  Genetic differences.  Environmental factors. 1 BIOLOGY & GEOLOGY 4th ESO 2.2. Types of characteristics Characteristics in humans can be grouped into three categories: continuous, discontinuous and blood groups. a) Continuous characteristics There is a minimal variation: differences between individuals are very small. Inheritance is polygenic and quantitative.  Polygenic inheritance: presence of several genes. Examples: ◦ Hair colour, skin colour, eye colour, height, longevity.  Quantitative inheritance: continuous traits that are regulated by independent genes. b) Discontinuous characteristics There is a large variation: differences between individuals are very big.  Inheritance: it consists in a pair of alleles or multiple allelism, or it is linked to chromosomes. Examples: ◦ Tongue fold in a U shape (d) or not (r). ◦ Thick (d) or thin (r) lips. ◦ Separated (d) or glued earlobe (r). ◦ Long (d) or short (r) eyelashes. ◦ Curly (d) or straight (r) hair. ◦ Presence of freckles (d) or absence (r). c) Blood groups There are two main blood groups: AB0 and Rh. The AB0 blood group has 4 blood groups (A, B, AB and 0), with some characteristics:  It depends on the presence of antigen A and antigen B in the erythrocyte membrane.  In plasma there is anti-A antibody and anti-B antibody.  If red blood cells match an antigen and the corresponding antibody, the blood clots: ◦ Antigen A + anti-A antibody = death. ◦ B antigen + anti-B antibody = death.  The characteristic is related to a gene on chromosome 9 that has three alleles: ◦ Allele A: responsible for antigen A. ◦ Allele B: responsible for antigen B. ◦ Allele 0: no information for any antigen.  The relationship between the alleles is as follows: ◦ Alleles A and B are codominant: people who present them have both antigens. ◦ Allele A and B dominate over 0, which is recessive. 2 BIOLOGY & GEOLOGY 4th ESO The Rh blood group has 2 blood groups (Rh+ and Rh-), with some characteristics:  It depends on the presence of antigen Rh in the erythrocyte membrane.  In plasma there is anti-Rh antibody.  If red blood cells match an antigen and the corresponding antibody, the blood clots: ◦ Antigen Rh + anti-Rh antibody = death.  The characteristic is related to a gene on chromosome 1 that has two alleles: ◦ Allele R: responsible for antigen Rh. ◦ Allele r: no information for any antigen.  The relationship between the alleles is as follows: ◦ Allele R dominates over r, which is recessive. 3. Genetic disorders Genetic disorders are changes in genes that express differences from the average individuals. There are three types: single-gene disorders, chromosomal disorders and genomic disorders. 3.1. Single-gene disorders The single-gene disorders are due to the existence of defective genes. Main types are:  Genes that cause illnesses: they led to organs and systems stopping working properly. ◦ Cancer: malfunction in a tissue caused by mutations on its cells that led to the growth of that tissue. It can spread to other parts of the organism (metastasis). 3 BIOLOGY & GEOLOGY 4th ESO  Autosomal disorders: located in the autosomal chromosomes. ◦ Autosomal dominant disorders: present in the dominant allele. ▪ Polydactyly: a condition in which an individual has one or more extra fingers. ◦ Autosomal recessive disorders: present in the recessive allele. ▪ Albinism: lack of melanin in the skin, leading it to be pale. ▪ Cystic fibrosis: condition that causes damage to the lungs, digestive system and other organs in the body. ▪ Sickle-cell anaemia: some red blood cells are shaped like sickles or crescent moons, becoming rigid and sticky, which can slow or block blood flow.  Sex-linked disorders: located in the differential segment of the X sex chromosome. The trait is more likely to be visible in the male. ◦ Haemophilia: rare condition that affects the blood's ability to clot. ◦ Colour blindness: inability to differentiate certain colours.  Inheritance influenced by sex: autosomal traits that are influenced by sex: if a male has one recessive allele, he will show that trait, but it will take two recessive for the female to show that same trait. ◦ Alopecia: loss of hair in the head. ◦ Waardenburg syndrome: conditions that can cause hearing loss and changes in colouring (pigmentation) of the hair, skin, and eyes. 3.2. Chromosomal disorders Changes in the structure of chromosomes. Main types are:  Deletions: loss of a fragment.  Duplications: an area of the chromosome is duplicated.  Inversions: a chromosome fragment is inverted.  Translocations: exchange of several fragments with each other. 3.3. Genomic disorders They affect the number of chromosomes because one is missing or duplicated. They are called aneuploidies:  Nulisomies: one missing pair of homologues → 2n-2.  Monosomies: one missing chromosome → 2n-1.  Trisomies: one more chromosome → 2n + 1. 4 BIOLOGY & GEOLOGY 4th ESO There are different types according to the affected chromosomes:  Autosomal disorders: they affect the number of the autosomal chromosomes. The most frequent ones are trisomies. ◦ Down syndrome: trisomy of the chromosome 21.  Sex chromosome disorders: they affect the number of the sex chromosomes. ◦ Alterations that affect women: they affect one of the X chromosomes. ▪ Triple X syndrome: trisomy of the chromosome X → XXX. ▪ Turner syndrome: monosomy of the chromosome X → X. ◦ Alterations that affect men: they affect the X or the Y chromosomes. ▪ XYY syndrome: XYY trisomy → XYY. ▪ Klinefelter syndrome: XXY trisomy → XXY. 4. Congenital malformations 4.1. Congenital malformations Alterations in the shape, structure, or size of organs during foetal development. Causative environmental factors are:  Radiation: X-rays, ionising radiation.  Chemical agents: chemical products.  Infectious agents: microorganisms. 4.2. Common congenital malformations Two common congenital malformations are spina bifida and cleft lip.  Spina bifida: a fault in the development of the spinal cord and surrounding bones (vertebrae) that leaves a gap or split in the spine. The spinal cord does not form properly, and may also be damaged.  Cleft lip: an opening or split in the upper lip that doesn't close fully when an unborn baby is developing in the womb. 5 BIOLOGY & GEOLOGY 4th ESO 5. Diagnosis of genetic diseases 5.1. Diagnosis of genetic diseases Early diagnostic tests: allows to prevent the disease from manifesting:  Cytogenetic tests: involve testing samples of tissue, blood, or bone marrow in a laboratory to look for changes in chromosomes, including broken, missing, rearranged, or extra chromosomes.  Biochemical tests: includes research done from a blood or urine sample, spinal fluid, or other tissue sample to detect abnormalities in a protein function. In newborns various health problems that can be found and treated before the child becomes sick.  Molecular tests: laboratory method that uses a sample of tissue, blood, or other body fluid to check abnormalities in DNA sequences that may be a sign of a disease or condition, such as cancer. 5.2. Common techniques Most common techniques in the diagnosis of genetic diseases are ultrasounds and amniocentesis.  Ultrasounds: diagnostic ultrasounds use sound waves to make pictures of the body. Also called sonography, ultrasounds can show organs and tissues.  Amniocentesis: a process in which amniotic fluid is sampled using a hollow needle inserted into the uterus, to screen for abnormalities in the developing foetus. 6