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Exceptions to Mendelian Genetics Some alleles are neither dominant nor recessive, and some traits are controlled by multiple alleles or multiple genes.

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Presentation on theme: "Exceptions to Mendelian Genetics Some alleles are neither dominant nor recessive, and some traits are controlled by multiple alleles or multiple genes."— Presentation transcript:

1 Exceptions to Mendelian Genetics Some alleles are neither dominant nor recessive, and some traits are controlled by multiple alleles or multiple genes and may be influenced by the environment

2 Incomplete dominance the heterozygous phenotype is somewhere between (intermediate to) the two homozygous phenotypes. We see this often in flowers.

3 incomplete dominance P generation parents are homozygous red and white All F1 offspring are pink and heterozygous When F1 offspring are allowed to self-pollinate, there is a 1:2:1 phenotypic ratio for the F2 offspring

4 Codominance Both alleles are expressed to some degree in the heterozygous phenotype Ex. “Erminette” chickens, roan cattle

5 Codominance In chickens, the allele for black feathers and the allele for white feathers are both being expressed. In roan cattle, the allele for red coat color and the allele for white coat color are both being expressed.

6 Codominance in Cattle

7 Multiple alleles More than two alleles may exist for a given trait. An example is coat color in rabbits. C - full (brown/gray) coat color - dominant to all other alleles Cch - “Chinchilla” coat color - all gray - dominant to Ch and c alleles Ch - “Himalayan” coat color - white with black features - dominant to c allele c - Albino coat color - no pigmentation - recessive to all alleles

8 Multiple Alleles Human blood types are the result of multiple alleles Three alleles: I A, I B, and i I A and I B are both dominant over i, neither is dominant over the other When the I A and I B alleles occur together, they are codominant

9 Polygenic traits Complex traits that are controlled by multiple genes. It can be difficult to accurately predict the appearance of some polygenic traits in offspring. Ex. human eye color, coat color in labs

10 Coat color in Labs

11 Qualitative v. Quantitative Traits Qualitative traits - have a clear quality, simple inheritance, limited environmental influence Ex. attached earlobes Quantitative traits - measurable but has many different qualities, complex, large environmental influence Ex. milking ability


13 Charles Darwin Introduction to Natural Selection

14 Darwin’s Journey In 1831, 22-year old Darwin leaves on a 5 year voyage as a naturalist on the HMS Beagle. The Beagle travels around the world, with numerous stops along the South American continent. As a naturalist, Darwin must make observations of the natural world, specifically plants, animals, geology, and climate of the various regions.

15 Darwin’s Journey

16 While on his journey, Darwin makes a number of observations: Animals living in similar environments exhibit some of the same adaptive features. Fossil remains of extinct animals resemble some current species.

17 Darwin’s Journey For example, fossils of the Glyptodon very closely resemble, but are not the same as the armadillo. This leads Darwin to believe that species must be changing over time.

18 Darwin’s Journey This notion that species change over time clashes with the beliefs of that time period. People are firm believers in divine creation - the belief that a divine being was responsible for the creation of all life forms on Earth. This view is static and does not allow for change.

19 Science in the 1800’s At the time, a comprehensive fossil record is being established by geologists. The fossil record contradicts the theory of divine creation. Several people in the science community begin to discuss species change.

20 Lamarck In 1809, Lamarck theorizes that: Physical features change in size based on how much or how little the animal uses them in it’s lifetime. Use and disuse. These acquired changes are heritable - are passed from parents to offspring.

21 Lamarck

22 Lamarck theorized that these sorts of traits were heritable. As we will soon see, evolution is something that takes place over generations, not in an individuals lifetime. Lamarck did lay the foundation for Darwin’s theory by stating that species change with the “physical conditions of life” - the species environment. These changes are evolutionary adaptations.

23 Darwin’s Journey Darwin returns from his journey in He has overwhelming evidence to show that evolution exists. He doesn’t publish his theory until Why?

24 Malthus Darwin finds the answer to his problem in an unlikely place - an essay on economics by Thomas Malthus. In the paper, Malthus is discussing the growth of human populations and food supply.

25 Malthus Due to different types of growth, populations could easily exceed resources. When resources become scarce, war and famine control the population. What does this have to do with Darwin and evolution?

26 Malthus To summarize the main point of Malthus’ essay, human populations are limited by their resources. Humans are limited by their environment. When humans come to conflict over scarce resources, the stronger, better-equipped will win. Sound familiar?

27 Natural Selection That’s because it’s the basis for Darwin’s mechanism of evolution! Darwin believes that this applies not simply to humans, but to all living species. Darwin theorizes that species evolve through the process of natural selection.

28 Natural Selection Natural selection is defined as the differences in survival and reproduction among individuals in a population as a result of their interaction with the environment. In other words, some individuals are better able to survive and reproduce because they have traits that make them better at coping with their environment

29 Natural Selection Two major premises for Darwin’s theory: Species are not immutable. The driving force behind evolutionary change is natural selection or “survival of the fittest”.

30 Natural Selection Within a population, genetic diversity exists. In a particular environment, certain individuals in a population are better suited to survive. Over time, genes that code for traits that enable individuals to survive and reproduce accumulate (greater allelic frequency). Evidence indicates modern species evolved from organisms that are now extinct

31 Evolution

32 Natural Selection Beneficial traits are evolutionary adaptations to the environment, and increase the individuals fitness - the relative ability to survive and leave offspring.

33 Role of mutation New alleles arise by chance out of random mutations. Some mutations better equip an organism to survive and reproduce (increase fitness), which will lead to those new traits being passed on. We consider those traits “beneficial” If environmental conditions remain the same, over time the beneficial trait will increase in frequency

34 Natural Selection Darwin presented the following arguments to support evolution: Populations have a huge reproductive potential Population sizes remain relatively stable. Resources are limited. Individuals compete for limited resources.

35 Natural Selection There is variation among individuals in a population. Most of this variation is heritable. Only the most fit individuals survive. Individuals with traits best adapted for survival in the environment are able to outcompete others. Over time, traits best adapted for survival in the environment (and the genes behind them) accumulate in the population.

36 Types of Selection Stabilizing Selection Directional Selection Disruptive Selection Sexual Selection Artificial Selection Natural Artificial

37 Directional Selection Case of the Peppered Moth in England Pre- Industrializatio n = light moth is selected for.

38 Directional Selection After the Industrial Revolution, thick soot covers the trees that the moths live on. Now, dark moths are selected for.

39 African Swallowtail

40 Sexual Selection Male Competition - Brooklyn Parrots

41 Male Competition

42 Sexual Dimorphism Peafowl

43 Sexual Dimorphism

44 Sexual Selection Female Choice - Blue-Footed Booby

45 Female Choice

46 Stabilizing Selection Eliminates individuals that have extreme or unusual traits. Individuals with the most common trait are best adapted. Height variation in humans adheres to stabilizing selection

47 Directional Selection Favors traits that are at one extreme of a range of traits. Traits at the opposite extreme are selected against. Example: Industrial Melanism

48 Disruptive Selection Selection for extreme or unusual traits, while selecting against the common traits. Example: weeds and disruptive selection for height.

49 Sexual Selection Differential mating of males (sometimes females) in a population. Male competition: awards mating opportunities to the strongest males. Female choice: leads to traits or behaviors that are attractive to females.

50 Artificial Selection Directional selection carried out by humans

51 Artificial Selection



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