Mindarie Senior College 2A/2B Human Biological Science Genetic Variation Mendelian Patterns of Inheritance

Gregor Mendel Born in the Czech Republic in Born in the Czech Republic in  Died in 1884 with his work still unnoticed. Worked in an Austrian monastery as a monk/teacher/scientist/gardener. Worked in an Austrian monastery as a monk/teacher/scientist/gardener. Experimented with purebred tall and short peas. Experimented with purebred tall and short peas.  Discovered some of the basic laws of heredity.  Studied seven pure bred traits in peas.  Called the stronger hereditary factor dominant.  Called the weaker hereditary factor recessive.  Presentation to the Science Society in 1866 went unnoticed. ie, no comtemporary recognition Work and findings rediscovered in 1900 by Hugo de Vries and Carl Correns Work and findings rediscovered in 1900 by Hugo de Vries and Carl Correns Known as the “Father of Genetics”. Known as the “Father of Genetics”. Moral: you too can be famous! Moral: you too can be famous!

Mendel’s Observations He noticed that peas are easy to breed for pure traits and he called the pure strains purebreds. He noticed that peas are easy to breed for pure traits and he called the pure strains purebreds. He developed pure strains of peas for seven different traits (i.e. tall or short, round or wrinkled, yellow or green, etc.) He developed pure strains of peas for seven different traits (i.e. tall or short, round or wrinkled, yellow or green, etc.) He crossed these pure strains to produce hybrids. He crossed these pure strains to produce hybrids. He crossed thousands of plants and kept careful records for eight years. He crossed thousands of plants and kept careful records for eight years.

Mendel’s Peas In peas many traits appear in two forms (i.e. tall or short, round or wrinkled, yellow or green.) In peas many traits appear in two forms (i.e. tall or short, round or wrinkled, yellow or green.) The flower is the reproductive organ and the male and female are both in the same flower. The flower is the reproductive organ and the male and female are both in the same flower. He crossed pure strains by putting the pollen (male gamete) from one purebred pea plant on the pistil (female sex organ) of another purebred pea plant to form a hybrid or crossbred plant. He crossed pure strains by putting the pollen (male gamete) from one purebred pea plant on the pistil (female sex organ) of another purebred pea plant to form a hybrid or crossbred plant.

Mendel’s Results Mendel crossed purebred tall plants with purebred short plants and the first generation plants were all tall. When these tall offspring were crossed the result was a ratio of 3 tall to 1 short.

Mendel’s Peas

Mendel’s Experiments He experimentally crosses different strains to develop hybrids. He experimentally crosses different strains to develop hybrids. He then crossed the hybrids and analyzed the results. He then crossed the hybrids and analyzed the results.

Dominant Traits RULE Strong Hereditary traits cover weak traits. Strong Hereditary traits cover weak traits. Mendal called stronger traits Mendal called stronger traits DOMINANT DOMINANT Mendal called weaker traits Mendal called weaker traits recessive recessive Dominant traits are represented by capital letters (T) while recessive traits are represented by lower case letters (t). Dominant traits are represented by capital letters (T) while recessive traits are represented by lower case letters (t).

Important terms used in genetics TermMeaning Monohybrid cross The inheritance of a genetic feature controlled by a single pair of genes. (monohybrid = an organism that is hybrid for one genetic feature) Allele The different forms of a gene. Most genes have only two alleles (e.g. the height of the pea plants in the above example was controlled by a gene, which had a tall allele and a short allele). Although some genes have more than two alleles a hybrid individual can only possess two of them.

Homozygous Where both alleles determining a genetic feature are the same (e.g. TT) Heterozygous Where the two alleles determining a genetic feature are different (e.g. Tt) Genotype The combination of genes that determine a character, whether or not they are expressed (e.g. Tt)

Phenotype The physical expression or appearance of a particular genotype (e.g. the physical expression or phenotype of a pea plant with the genotype Tt will be a plant that is tall). Autosome A non-sex chromosome Sex chromosome (X-chromosome) The 23rd pair of chromosomes that determine the sex of an individual

Mendel’s first law - The Law of Segregation This law states that: allele pairs separate or segregate during gamete formation, and randomly unite at fertilization allele pairs separate or segregate during gamete formation, and randomly unite at fertilization

The underlying assumptions The Law of Segregation is based on the assumption that: There are alternative forms for genes (known as alleles) There are alternative forms for genes (known as alleles) For each characteristic, or trait, organisms inherit two alternative forms of that gene - one from each parent For each characteristic, or trait, organisms inherit two alternative forms of that gene - one from each parent When gametes (sex cells) are produced, allele pairs separate leaving the gametes with a single allele for each trait When gametes (sex cells) are produced, allele pairs separate leaving the gametes with a single allele for each trait When the two alleles of a pair are different, one is dominant and the other is recessive When the two alleles of a pair are different, one is dominant and the other is recessive

Mendel’s hypothesis - 1 Mendel predicted that if he cross- pollinated true-breeding long-stemmed plants (homozygous dominant) with true- breeding short-stemmed plants (homozygous recessive), all of the resulting offspring (the F1 generation) would be long-stemmed. Mendel predicted that if he cross- pollinated true-breeding long-stemmed plants (homozygous dominant) with true- breeding short-stemmed plants (homozygous recessive), all of the resulting offspring (the F1 generation) would be long-stemmed.

Dominant allele (long stem) Recessive allele (short stem) TT T tt t Tt The gametes or sex cells Tall Short

Calculating autosomal dominant- recessive patterns of inheritance Let the allele for long stems be represented by T Let the allele for long stems be represented by T Let the allele for short stems be represented by t Let the allele for short stems be represented by t Parental phenotype Tall x Short Parental genotype TT x tt Possible gametes T x t only F1 genotype All Tt F1 phenotype All tall

Punnett Squares Predicting the result of a genetic cross is made easier by using a Punnett square. Predicting the result of a genetic cross is made easier by using a Punnett square. T T t t TTTt tt Genotype of first parent Genotype of second parent Predicted genotypes of offspring

* Statistical predictions The result of genetic crosses only shows the statistical probability of an offspring having a particular genotype The result of genetic crosses only shows the statistical probability of an offspring having a particular genotype

Example: Example: The chance of a baby being a particular sex is ½ male: ½ female (or 1:1). This means that EVERY baby has an equal chance of being a boy or a girl – if the first baby is a boy the second baby has a 50% chance of also being a boy. The chance of a baby being a particular sex is ½ male: ½ female (or 1:1). This means that EVERY baby has an equal chance of being a boy or a girl – if the first baby is a boy the second baby has a 50% chance of also being a boy. The predicted ratio can be seen best in large populations, such as Mendel’s pea experiments, which involved thousands of plants. The predicted ratio can be seen best in large populations, such as Mendel’s pea experiments, which involved thousands of plants.

Some Mendelian traits in Humans Ability to taste PTC Ability to taste PTC Albinism Albinism Blood type Blood type Brachydactyly (Shortness of fingers and toes) Brachydactyly (Shortness of fingers and toes) Cheek dimples Cheek dimples Cleft chin (cleft=dominant; smooth=recessive) Cleft chin (cleft=dominant; smooth=recessive) Free or attached earlobes Free or attached earlobes Wet or dry earwax Wet or dry earwax Face freckles Face freckles Hitch hiker’s thumb Hitch hiker’s thumb Polydactyly (six fingers) Polydactyly (six fingers) Widow’s peak (Widow's peak=dominant, straight hairline=recessive) Widow’s peak (Widow's peak=dominant, straight hairline=recessive) Tongue rolling Tongue rolling