Lecture 12 - Inheritance, characters and traits.pdf
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Inheritance, characters and traits Coley Tosto [email protected] (adapted from Dr. Amy Osborne) Learning objectives Demonstrate a strong understanding of terminology around chromosomes, alleles Explain the relationship between genotypes and phenotypes in dominant and recessive gen...
Inheritance, characters and traits Coley Tosto [email protected] (adapted from Dr. Amy Osborne) Learning objectives Demonstrate a strong understanding of terminology around chromosomes, alleles Explain the relationship between genotypes and phenotypes in dominant and recessive gene systems Describe how pedigrees can help us understand inheritance systems Explain X-chromosome inheritance, and sex-linked disorders Describe dominant lethal inheritance patterns Interactive task – build a glossary! Introduction Homologous chromosomes Our genes are contained on two homologous chromosomes Each chromosomes has the same genes (one maternal, one paternal copy) Two copies of the same gene are called alleles The alleles may code for different versions of the same trait/characteristic. Most genes have more than two alleles Phenotypes and genotypes Genotype Phenotype Alleles interact with each other to form our traits Dominant and recessive alleles A = dominant allele a = recessive allele Dominant and recessive alleles Dominant Traits Recessive Traits Achondroplasia Albinism Brachydactyly Cystic fibrosis Duchenne muscular Huntington’s disease dystrophy Marfan syndrome Galactosemia Neurofibromatosis Phenylketonuria Albinism – multiple Widow’s peak Sickle-cell anemia genes, e.g. TYR gene Wooly hair Tay-Sachs disease Pedigrees You need two copies of a recessive gene to display the trait/disease Some people carry the gene without being affected Pedigrees help us work out whether we carry a disease-causing gene Alkaptonuria – recessive disorder. You can work out a parent’s genotype by looking at their offspring. X-linked traits Sex chromosomes are non-homologous The Y chromosome has a small homologous section (so the chromosomes can pair during meiosis) The Y chromosome doesn’t have many genes on it Males are hemizygous (only one allele of an X-linked characteristic). Human sex-linked disorders Red-green colour blindness and some types of haemophila are sex-linked disorders. Both are recessive traits X-linked disorders are much more common in males No father-son transmission Females must inherit two copies A female with one copy is a ‘carrier’ Carrier females can pass the trait to their male children, and their daughters can be carriers. Lethality – recessive lethal alleles Most genes are essential for survival If an allele is recessive (non-functional gene) it can remain in circulation because individuals have a functioning copy. If two parents are heterozygotes (one functional, one non-functional allele) 25% of their offspring would be homozygous recessive If the gene is essential to life, the individual may die This is called a ‘recessive lethal’ inheritance pattern Lethality - dominant lethal alleles Sometimes a single functional allele is not enough! ‘Dominant lethal’ inheritance pattern – lethal in heterozygous form as well. Very rare but can be passed on if the lethality isn’t expressed until adulthood Huntington’s disease – nervous systsem disorder Huntington allele (Hh) heterozygotes Onset often not until after 40 years of age – 50% of offspring may have the gene by this point.