Lecture 12 - Inheritance, characters and traits.pdf

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.